Where is the best place in Europe/UK to Hot Air balloon?

You can't really compare prices for a cruise over England with a cruise over Southern France with a cruise of East Germany. A trip over a big city is something different than a trip over some rural landscape. They all stand on their own, so do the prices.

Note also that a ballon heavily depends on the current weather conditions. They won't start in rain, they won't start if there's more wind than a slight breeze. You'll only learn for sure if they start between 10 down to 3 hours beforehand.

It might therefore be a good idea to not book the least expensive flight but wherever you plan stay the longest time.

Nevertheless:

Rides over Germany are around 180 € to 190 €. There's a comprehensive list at http://www.my-skyworld.de/index.php?id=118

A list for the UK can be found at http://www.balloonride.org.uk/ , prices seem to vary between £100 and £180.

The Tavistock hot air balloon festival Tavistock Devon England, they fly over Dartmoor and is a major annual ballooning event.

Tuscany would be an excellent choice: http://www.flyballoon.it/, http://www.balloonintuscany.com/,

http://www.vastanomongolfiere.com/it/

or this one near Paris is good: http://www.hotairballoon.fr/