Try going thru the UK websites for Avis, Hertz and Alamo etc and see what prices you get for comparison ( not forgetting to convert from GBP to USD of course ).
Car hire in the US tends to be inflated by the excessive additional amounts they charge per day for insurances/collision damage etc ... the UK sites don't charge for this separately and generally give an all-inclusive price. You'll have to pay in Sterling but using a credit card that shouldn't be a problem.