I think Spain was already a second rank power long before the Napoleonic wars. I would say it had been so for a century. Spanish dominance ends for me somewhere in the begining of the 17th century (the invincible armada being destroyed during the late 16th century, and the equally invicible tercios beaten in the middle 17th century being somewhat arbitrary but significant events). After that, no offence intended but I think that Spain begins to go down the toilets and that by 1700 it’s no more what it used to be.
Also, if you intent to list majors European powers, you forgot the Austrian empire and latter Russia.
However, regarding the OP, during this period of Spanish decline, Britain is on the rise but doesn’t have yet reached a dominating status. The British army isn’t very relevant (and in fact mostly never has been until the 20th century, American beliefs about their revolutionary war notwithstanding ;)), even her main asset, her navy, isn’t yet “ruling the waves” (before 1700 the British navy is only roughly on par with France and…Holland) and technical improvements (that would be the main cause of British power. You don’t get to be the big guy on the block without a strong economy) only begin to be noticeable in the mid 18th century (noticeable in the sense that it already appears a bit more advanced than other European countries. This advance will become quite strinking during the early 19th century).
I agree with a previous poster. The “British era” could be dated from the Napoleonic wars until WWI, so, essentially, the 19th century, by which time Spain was a quite backward country in Europe.
Generally, I’d say that European dominance begins with the Renaissance and ends with WWI. And in very rough terms I’d divide this era in the following way :
1500-1650 : Spanish dominance (arbitrary dates : 1516-1643)
1650-1800 : French dominance (arbitrary dates : 1643-1815)
1800-1900 : British dominance (arbitrary dates :1815-1918)