I'm not 100% sure as too if the company I'm about too recommend offers toll-free UK numbers or not but I'm quite confident that they do. Check out RingCentral - I've been with them for a little over a year now and do not have nothing negative to comment about regarding them.
Honestly, Asterisk IS extremely easy to use.. Even for a novice. Install & Setup was a breeze. I'm not too sure about termination services offered in the UK. I'm sure there are companies that offer services for an affordable price in the UK as there are in North America.