London: Greatest City in the world!
Greater London has great mass-transit (buses, trains, underground "tube", etc). Also there are 5 major airports in the general London area (Heathrow, Gatwick, Stansted, Luton, London City). Many budget airlines operate within the UK and goto mainland Europe.
There are essentially 6 concentric circles called zones. Central London (zone 1) and then another progressive zone outside, and another, etc... so zone 6 is basically outer/suburban London. Allday, unlimited rail/underground/bus ticket called "Travel Pass" is 6 GBP (~$10) and it's good for within Zone 1-6. Try to stay in Central London, it'll make things more convenient! But it's not a must, if costs are prohibitive (again, for what you spend... you don't get much in return; relatively speaking)
Hotels are pricier than the US, and w/ less comfort (for the most part). You can stay in any zone and be in Central London via rail/underground in 30'ish minutes at most (depending on rush & whether direct route). Train stations are VERY easy to get to, and generally are everywhere, so to speak.
Check-out:
http://www.what-to-do.org/ (London Hotels!)
http://www.lastminute.co.uk
http://www.airlinenetwork.net
http://www.nationalrail.co.uk
I find the links above to be better than Expedia, Orbitz, etc...
And of-course:
http://www.nsxcb.co.uk
If you fly into Heathrow, take the train/underground into London {approx 45mins}; NOT a cab.
If you fly into Gatwick, take the Gatwick Express train/underground into London {approx 1 - 1 1/2 hrs); NOT a cab or coach (shuttle-bus).
There are two types of cab service: black cab (expensive, but can be solicited w/o advance call) & mini-cab (economical, you need to book one in advance via phone as they can't be solicited/hailed).
Car rentals aren't overly expensive. Compare deals online. Driving into Central London, there is a congestion-charge (basically a tax to persuade less vehicular traffic into city); also parking is nearly non-existent.
Here's a map that can offer more insights, familiarize yourself w/ it...
