Just use ATMs and banks, carefully count your change until you get used to the local cash, and stay away from gray market money changers
This is not the hint that you should use an ATM at a bank over an exchange bureau (and, in decreasing order of beneficial exchange rates, a hotel exchange desk over a shop).
This is about avoiding the blatant money-changing thieves.
Some unscrupulous types try to catch new arrivals and their initial confusion over unfamiliar money to shortchange you.
Until you get the hang of the currency, they mighy look pretty similar (in that neither looks instantly familiar), and unless you take a sec to see which one you're taking out of your wallet, a slick con artist might glibly insist you gave him a different one (he'll even show you "your bill," which is, of course one he keeps in reserve for just such a slight-of-hand scam).
Until you get used to the money, examine each bill carefully before you hand it over, and make a show of doing so.
Also, familiarize yourself with the current currency in circulation at Bankofengland.co.uk.
The walking, talking ATM and his light fingers
This is two scams rolled into one—or, technically, a scam wrapped in a pickpocketing. First, the scam: A stranger offer to help you change money. Alarm bells should go off immediately.
While in some countries, buying currency on the gray market like this can work in your favor (but you have to know what you're doing), those countries are no longer found anywhere in Europe—certainly not in Britain. No dude on the street can give you a honest rate that's better than you'd get at a bank. Besides, this is one major way locals dispose of counterfeit bills. Just say no.
OK, say you skipped that last paragraph and fell for the scam anyway—we'll even give you a lame excuse for it: it's 3am, you just got off the train in a new town, you have nothing but some emergency greenbacks, everything is closed, the ATM in the station isn't working, and the only hotel in town demands cash up front. (Yes, this has happened to me—though to be fair, it was in Poland, not Britain, and this happened pretty soon after the fall of the Berlin Wall, so things weren't really running well yet there.)
So you dabble with the dude and his gray market money. He is a warm and friendly guy, your money-changing savior, and after the deal is concluded, he goes all Mediterranean on you exclaiming, "You are my new American friend!" And wraps you in a big bear hug by way of goodbye. You stand there, bemused by his effusiveness, and he walks away with a wave and a smile.
Congratulations, he switched gears from con artist to pickpocket and just stole back the money he gave you, plus whatever else was in your wallet.