I went with Travel Continental because a girl I worked with was with the the entire time she was living in London. Because they do 3 month placements, you're not tossed around different hospitals (in case that's not your thing!). I haven't started working yet, just started the ONP course yesterday, but Travel Continental have been really efficient in helping setting me up. I went by their office the other week, which seems like a busy, noisy place
if you contact them, they'll send you a massive email with all the details.
The 250-300 (3000 pounds per year) allowance for living is true, in case you don't want to live in a shared accommodation with 2-3 other nurses! Other financial details as of last year (not sure if anything has changed...) - travel allowance of 175 pounds per 13 week contract = 700 pounds per year; they'll reimburse your RCN at 193; holiday pay (with 4 13 week contracts = 2430 pounds...)