Never use your credit card to obtain money (either abroad or in the UK), you will be charged an up front fee of around 3%, even if you payoff everything at the end of each month.
Credit cards are OK for purchases, again you will do no worse than the Post Office here. Be aware that most gas stations charge extra for using credit cards, and on one of my trips to Hicksville USA, no bank would exchange my Sterling for US$ unless I had an account with them.
On my recent trip to Las Vegas, the Post Office here was offering a rate of $1.55, at my hotel the rate was $1.60. I assumed the rate had improved, as hotels are not usually known for offering good exchange rates. I checked on my return, and the rates had remained pretty constant.