we know from history that the Spanish Empire almost succeded in invading the UK in 1588 had it not been for freaky weather.
Up til 1588 was a major Naval Power in Europe and as History Channel claims more powerfull than the UK at that time.
I wonder is it possible with a p.o.d. to make the UK the powerfull naval empire at that time?
Maybe having Elizabeth the first wanting to invade Spain in 1588?
One nitpick before I respond to your question. Your use of UK is wrong. Only England was under threat, and Scotland wouldn't be united with England until after Elizabeth's death, and even then it was only personal union. The United Kingdom of Great Britain only exists after 1707.
Now to the question at hand:
Spain WAS more powerfull than England at the time. Population wise, it had around a million more people in it, along with the wealth of the Americas in it.
The Spanish Armada was massive for the time. Everyone was awed by it, but it was damn expensive. And I mean expensive. This fact itself doesn't lend itself well to the fiscally conservative Elizabeth, who I don't think would be caught dead wasting that much treasury money to invade a solidly catholic Spain.
Religious reasons are also a problem. Spain was solidly catholic for a number of reasons. One, the popular piety movement of the 15th and 16th centuries took the wind out of any protestant sails, and two, catholicism was associated with victory over the moors (a very proto nationalism in post reconquista Spain). England on the other hand was not solidly protestant. When Elizabeth inherited the throne, the majority of Englishmen were still catholic (this would change by the end of her reign). The English monarchy has nothing to gain by bringing more catholics under their wing.
The gist of what I'm saying is that you would never catch Elizabeth actually trying to conqer Spain. She didn't have the money to do it and even if she had, she still wouldn't invade. Religiously, Spain had cause and England didn't. English protestantism was all about of securing itself from Papal control and influence, while Spanish Catholicism was still caught up in the crusading reconquering spirit.
I'm not saying that an English Armada couldn't be launched in the late 16th century, but you need an earlier POD. My best bet would be some kind of
semi victory in the Hundred Years War with England somehow inheriting the wealth of Burgundy instead of the Hapsburgs.