FNG is right, all hours count - from your very first trial flight, providing of course, you are Pilot in command or under training. Being just a passenger doesn’t count!
Hour building is a case of getting as much valuable experience for as little money as possible.
To achieve this many people go abroad and combine it with a holiday. Many US schools offer 100hrs for an attractive fixed price - this means spending about 4 weeks abroad, which most people are unable to do.
By the time you have done this, bought plane tickets, added on living expenses etc, you may as well have bought a share in a UK a/c, hour built in your spare time and then sell the share and put it towards further training.
IMPO I believe that flying in the complex and congested UK airspace, dodging around British weather, is more useful than flying straight lines in Arizona for 200nm, but hey...
Money wise? A UK share from £1000 to what ever you want to spend. Look at a group charging no more than say £45/hr with fuel. Shares include a monthly payment in the order of (for a sensible hour building a/c) £20 -£80 per month.
Compare this to a US school and it's favourable, honest!
Bodie
Edited: FlyingForFun's advice is all good. Especially note his night rating comments.