Tbh, ATM will certainly be your easiest option, however, if you're planning to pick up a 'few supplies' like soda, snacks etc from a grocery store then it might be worth hitting somewhere like Tesco's and pay with your debit (not Credit) card as I believe they still offer a cashback service (gotta admit, I don't use it myself but they did last time I heard). I think that might have a £50 max on it per transaction however you would avoid the ATM usuage fee this way and could be ok for pocket money purposes.
You didn't say if you were staying in London or not, but if you are and it's central, there are a number of small exchange booths that offer pretty competitive rates commission free for cash exchanges but that would mean bringing cash $ over with you which kinda defeats the purpose.
There are also a number of places like the Post Office and a chain store called Marks and Spencer who offer 0% commission on Travelers Checks or debit cards but if their £ -> $ exchange rates are anything to go by, they're not very competitive so they just claw back their fees that way. However, they are as reputable and safe to use as any bank.
Just a heads up too. Most of our ATM's are free to use, certainly the obvious bank ones, however beware the smaller machines you might find in pubs or very small stores as they often also charge £1-2 fee for using the machine. It is however law that you're advised before you are charged so just make sure you're on the alert for it.
Hope that helps, enjoy your trip!