When did tea become "a thing" in England?

The history of tea, the meal, or teatime is indeed rather short compared to the English obsession of trading with it.

Afternoon tea, that most quintessential of English customs is, perhaps surprisingly, a relatively new tradition. Whilst the custom of drinking tea dates back to the third millennium BC in China and was popularised in England during the 1660s by King Charles II and his wife the Portuguese Infanta Catherine de Braganza, it was not until the mid 19th century that the concept of ‘afternoon tea’ first appeared.

Afternoon tea was introduced in England by Anna, the seventh Duchess of Bedford, in the year 1840. The Duchess would become hungry around four o’clock in the afternoon. The evening meal in her household was served fashionably late at eight o’clock, thus leaving a long period of time between lunch and dinner. The Duchess asked that a tray of tea, bread and butter (some time earlier, the Earl of Sandwich had had the idea of putting a filling between two slices of bread) and cake be brought to her room during the late afternoon. This became a habit of hers and she began inviting friends to join her.

Ben Johnson: "Afternoon Tea"

And this is Anna Russell, Duchess of Bedford:

The Duchess is best remembered as the creator of afternoon tea whilst visiting the 5th Duke of Rutland at Belvoir Castle in the mid-1840s. During the 18th century, dinner came to be served later and later in the day until, by the early 19th century, the normal time was between 7:00 and 8:30 p.m. An extra meal called luncheon had been created to fill the midday gap between breakfast and dinner, but as this new meal was very light, the long afternoon with no refreshment at all left people feeling hungry. She found a light meal of tea (usually Darjeeling) and cakes or sandwiches was the perfect balance. The Duchess found taking an afternoon snack to be such a perfect refreshment that she soon began inviting her friends to join her. Afternoon tea quickly became an established and convivial repast in many middle and upper class households.

Or more precisely:

Until 1794 tea was heavily taxed and smuggling was rife. It was smuggled ashore from Dutch merchant ships anchored off the English coast. Underground passages led from caves to unfrequented roads, providing a nationwide distribution network. The smugglers were put out of business when Prime Minister William Pitt the Younger drastically cut the tax on tea from 129 per cent to 12.5 per cent and tea became affordable to all social classes.[…]
By the middle of the eighteenth century tea-drinking had spread to the middle classes and replaced ale for breakfast and gin at other times of the day. As coffee-drinking declined tea became Britain’s most popular beverage. It was not the cost which caused coffee’s decline. Tea was more expensive per pound, although this was compensated for by the smaller amount required to brew the tea. A possible explanation is that to brew tea is easier, all that is needed is boiling water; coffee, in contrast, requires roasting, grinding and brewing. This could be done in the coffee houses but not so easily at home. The manner of trading may also have played a part. The tea trade was controlled by the powerful East India Company, whereas the coffee trade was run by independent merchants.

Fashions change, and with the expansion of London in the early nineteenth century the tea gardens closed down. Tea-drinking became confined to the home. Great social changes were also taking place. The main meal of dinner, previously taken in the middle of the afternoon, shifted to much later in the evening, sometimes as late as 8 or 9 o’clock, and only a light lunch was taken midday. Anna Maria, the Seventh Duchess of Bedford, is said to have experienced ‘a sinking feeling’ in the middle of the afternoon during the long gap between luncheon and dinner. She started taking a pot of tea with some light refreshments in her room mid-afternoon. She then began to have tea sent to her boudoir for herself and guests. In addition to a selection of teas (both China and Indian – the first Indian tea arrived in 1839 and Ceylon teas soon after in 1879), the Duchess served toast and fine breads. The tradition of afternoon tea was born. By the 1850s afternoon tea began to be served in the drawing room and became more elaborate and the focus for social visits. Dainty sandwiches, biscuits, cakes and pastries were served. The portions were usually small – just enough to stave off hunger pangs until dinner time. In winter, teas such as Assam with its rich pungent flavour and warming foods such as cinnamon toast, hot buttered crumpets and rich fruit cakes might be served round the fireside; in summer, delicate Earl Grey or the golden taste of Ceylon accompanied cucumber sandwiches and scones with strawberries and cream.

Helen Saberi: "Tea. A Global History", Reaktion Books: London, 2010.