Why were British ships not of the same quality as French and Spanish ships until the latter part of the 18th century?

I think that N.A.M Rodger covers this quite well in chapter 27 of his book "The Command of the Ocean".

It was for long an article of faith among naval historians that eighteenth century British warships were inferior to their French and Spanish opponents, because British shipwrights remained wedded to craft traditions while their continental rivals were men of education who applied mathematics and science to the solution of their problems. This judgement flattered, and sometimes still flatters, a range of agreeable prejudices. It fitted the eighteenth-century upper classes' admiration for France as the home of social glamour and prestige. It expressed British sea officers' conviction that as men of honour they were both morally and practically superior to civilian technicians; it magnified their courage and judgement when they won, and excused their failures when they lost.

That is the British of the time had good reason to be overly critical of their own vessels and the British have always seen glory in being the successful underdog. However, a key consideration is the purpose to which the ships were put:

Moreover the comparision between 'good' French and 'bad' British design rests on the naive assumption that the two were directly comparable, that British and French designers were building the same size and types of ship, to fulfil the same functions - in other words that the strategic situations of the two countries was the same.

and Rodger adds,

The proper question to ask of all ship designs is not how well they compared to one another, but how well they corresponded to each country's strategic priorities, and how wisely those priorities had been chosen.

That can be seen in how the British deployed their ships compared to the French and Spanish. As the century passed, the British aimed for what Mahan called "Sea Power", ensuring that their merchant fleet was protected and that their enemies' fleets were not free to move on the oceans. This required vessels that were able to stay at sea for long periods in all weathers. By comparison, the French and Spanish deployed their ships for specific missions, keeping them sheltered in port when not otherwise required. So their vessels didn't require the endurance of the British ships. The strategic priorities would determine the number of ships needed and their roles. The British approach needed lots of ships so even if the rivals had the same levels of naval expenditure, the British would have to spend less per ship (in both construction and maintenance) than the French and Spanish.

The fighting ability of a sailing warship is not simply a function of the vessel itself. The quality of the officers and crew were just as important (and were possibly more important) in determining how each individual vessel performed. In fact, the same ship with a different crew might handle very differently.

Another factor in how an ship handled is how the ship was loaded and trimmed. A ship just out of port on a long mission with a full load of stores and provisions with a fresh crew would be a different proposition to the same vessel on the return being lightly loaded with an experienced crew.

I think some care needs to be taken when talking about the idea "that British ships were sometimes built...following the plans of the beautiful and fast sailing French or Spanish ships". When foreign warships were captured, they were handed over to naval architects who made detailed plans of the vessels (Many surviving plans of French and Spanish vessels only exist because of this practice). If the warship was seen as worthy then the British often made a copy based "on the lines" of the foreign ship. This means that they copied the hull form (shape) rather than it being an exact copy in all details. The British still used their own construction methods to build the ship and they followed their own standards for fitting out the vessel. Given the primitive state of scientific understanding of what made a good ship, this approach makes sense. Copying a good working design was cheaper and less risky than developing a new one from scratch.

In addition, mention of the 'Forty Thieves' in this context is misleading since these vessels were built and put into service in the early nineteenth century. The issues with these ships had more to with the particular circumstances of their procurement than with general standards of ship design and building of their time.