How did the general population of England convert so smoothly to Protestantism?

It wasn't smooth and there were strong undercurrents both ways.

I just finished reading Ken Follett's book A Column of Fire', which although historical fiction, it covered many of the historical facts and highlighted a major difference between Mary and Elizabeth.

Mary had many executed by the inquisitors because they refused to believe as Rome did, ie the protested and paid with their lives.

Elizabeth, on the other hand, held the view that no-one should be executed for their religious belief, and regarded it as a matter of private conscience.

Many of those executed under her reign were found guilty of treason, mostly because they wanted to unseat Elizabeth in favour of a Catholic monarch.

Other contributors above have said there was little doctrinal difference until after Elizabeth's ascension. I suggest that the fundamental difference of 'justified by faith in the risen Lord' vs 'justified by obedience to Rome' has been around for many centuries, and persists until today, albeit in more subtle forms. Under Elizabeth, people were free to choose; under Mary, they were not.

I'm not sure I'd really describe it as either "smooth" or "peaceful", given that Catholic-Protestant rivalry can still divide families and is still capable of producing street riots in Britain today. Usually around the "Old Firm" football matches, but occasionally elsewhere.

There are really two factors to the Reformation, and church practice is only one of them. Loyalty is the other. At the start of the process, the church was unpopular enough (see Luther's 95 theses) that people were glad of change - and the opportunity to participate in looting and vandalism. I think people forget the huge ISIS-style erasure and destruction of religious art that happened in this period. Not to mention the martyrs of both sides being publicly burnt at the stake.

But each incoming monarch cared primarily about loyalty and the maintenance of absolute rule. The question was not so much one of religious belief as whether a given person would support the monarch by adopting their religion. Failure to do so was effectively treason. Changing religion easily was a matter of simple survival.

There's a song about this, The Vicar of Bray.

(This discussion leaves out Ireland and Northern Ireland. Ireland remained Catholic, and Northern Ireland had a sectarian civil war in the 20th century!)

See also: https://en.wikipedia.org/wiki/List_of_Protestant_martyrs_of_the_English_Reformation vs https://en.wikipedia.org/wiki/List_of_Catholic_martyrs_of_the_English_Reformation

Have you heard of the Pilgrimage of Grace? This was a major rebellion against Henry's move to break with Rome. The execution of over 200 of the participants was probably a very effective incentive for others to accept the change.

As others have said, for many the 'conversion' was a matter of convenience and survival. When Mary I came to the throne a substantial portion of the population reverted very happily to the old practices.

Not only was it not totally smooth, but it also wasn't much of a change. At least not on personal human timeframes.

You have to realize that the break in England didn't happen because anybody had any kind of doctrinal issue with Rome. King Henry VIII was not a protestant, did not like Protestantism, and did not want protestants in his Church. The only part of Catholicism he ever had any trouble with was the part where he wasn't allowed to do what he wanted.

At first you would have to have been part of the upper Church hierarchy to even notice a difference. The head of the Church of England no longer reported to the Pope, but rather served at the pleasure of the Monarch. The rites themselves didn't change all that much. The split between the two was always much more about politics than about religion.

It wasn't until the excommunication of Elisabeth I, 40 years later, that there was any truly significant doctrinal break between the two. Prior to that there were still CoE Bishops openly talking about reconciliation, or even alternatively joining with the Eastern Orthodox rite, as viable future options. People who held more typical European "Protestant" beliefs were not accomodated in the Church and were forced to become religious dissenters (eg: Puritans).

That papal bull was significant, because it was designed to empower an active rebellion against the Monarch. Effectively, it turned the political dispute between Rome and London into an active threat to the State. What followed was a couple of hundred years where the Catholic hierarchy was trying various ways to depose the leadership in England, and thus being a supporter of theirs was tantamount to treason.

So the main bone of contention was never really religious, and the practice didn't change all that much. The most obvious thing, performing mass in English, Catholic churches in English-speaking countries have changed to doing anyway. So even today, a lot of the more "high church" Anglican services would feel very familiar to a modern Catholic.

Well, it wasn't smooth.

First of all, there was already a minority of "reform" viewpoint in England before Henry VIII. It was centered in the intelligentsia and gentry.

So when Henry VIII decided to divorce the Church to marry Anne Boleyn, a significant and influential minority not only was in favor, but wanted to go further, faster.

And, as always, a big group of people just wanted to get on with their lives and hoped it would all go away.

Henry VIII's extravagance had bankrupted him and the country (there was not much distinction between the two at that time) so his other motive (besides Anne) was the wealth of the monasteries. By plundering them, he restored his checkbook's balance. Key point: He didn't keep the church lands, but sold it for ready cash to the gentry. Now, the gentry -- whose lifestyle revolved around the land -- had an incentive to support the new regime, since a return to the old would probably mean the disgorgement of all that lovely land. "Return to the True Church? Of course I'm in favor! Always have been. But those monasteries were pretty bad -- do you know what I saw on Facebook the other day? They were doing terrible things! -- Maybe we should restore the Church, but leave the lands alone. So that the Church can focus on saving souls, of course."

Once Queen Elizabeth took over from her activist siblings, she ruled as a kinder, gentler Protestant and -- mostly -- didn't punish people for what they believed, but for what they did. "Be Catholic if you like. Just be discreet about it, don't support the Spanish or the French, or Mary Queen of Scots, and don't expect royal favor." It wasn't a happy time for Catholics, but it was -- mostly -- tolerable as long as they avoided religious politics.

By the time that the Stewarts came in and made things a bit more difficult, this state of affairs had been in place for most people's whole lifetimes. It could be lived with. And it was.

But Catholicism never died and was a significant minority religion throughout.