The British Isles might not be the great power it would become yet but they could still tip the balance of a war fairly well. Just two years before the Restoration England made large gains against Spain, acquiring Jamaica, the Cayman Islands, Dunkirk and Mardyck (in Northern France)and in 1654 trounced the Netherlands in naval battles. The war between England and Spain didn't officially end until 1660 so I can see a Habsburg bride being offered to sea the peace. Actually I'm surprised that one wasn't in OTL.
That's because the only Habsburg princess avalaible both in Austria and Spain at the time (1660) was Margaret Theresa of Spain, and she was only 9 years old.