My fiance works for British Airways so I've flown direct to LHR and done europe more times than I can count. 10 days is a short time. Don't over do it in terms of destinations because it will ad costs and stress travel when it comes to bus / plane / Airbnb. Also, the northern part of Spain is awesome. In 12 days including travel from San Diego, we did Bilboa, Barcelona, and Madrid. I think France is super overrated and everyone there is a prick. Everyone also smokes. I have zero desire to go back to Paris. Just my 2¢.
In short, just pick two or 3 spots in Span and just do Spain. Then if you want to do France / Paris later make that a second trek. Or you simply need to increase your 10 days to 15 days.
Italy, Spain, and Ireland are my 3 most favorite Euro places. The eastern block of Europe sucks. Commies love concrete is pretty much all you need to know. Romania sucks. Prague is so touristy and overrated. Saw it all in one day and have no desire to go back. Lots of war history there. Some great views.
London obviously has tons of history, but the weather is the worst on the planet. It's worth doing, but again I think it's a pinch overrated. Cameras everywhere and in central London it's just like being in NY.
Germany is bad ass too.
That should give you some ideas should you want to extend your trip.