SOP here is to bung the names of the big players - Hertz, Avis, Budget, National, Alamo, Dollar, Budget - and the required pickup point into a Famous Web Search Engine and write down the numbers. Then book the cheapest using a different browser and/or Babbage-Engine, because cookies.
Read the small print to see what's included, especially Collision Damage Waiver. Paying at the time of booking is usually cheaper than paying when you pick the car up.
Age limit is usually 25 and no sneaking into Transmuralia, though Canuckistan is fine.
In theory you need the link to and/or the printout of, your driving record from the DVLA but no-one's ever asked me for that.
Some companies have a limit on the length of a single rental, usually 28 or 35 days. Going via the local website rather than the .co.uk version sometimes circumvents this.
Take the option to bring it back full of fuel, as they'll charge about twelve dollars per titchy USAnian gallon if they do it for you.
Become very VERY British if stopped by the police