Coinfloor seemed pretty solid (once you get through the KYC, etc) but they just lost access to Faster Payments.
Localbitcoins is probably the easiest - try cash (in person) purchases, they tend to be cheaper.
You could always try here (in the currency exchange sub forum).
Mycelium app is another option, not sure how popular it is though. Worth a shot.
Localbitcoins transactions in cash can attract a humongous premium.
I've used bittylicious the most. It's very quick and painless.
You have to keep a careful eye on sites like that though. Bittylicious prices completely depend on the seller at the time. Sometimes they're very close to exchange prices, other times they're ludicrously over.
bittybot.co.uk has a good overview of UK availability.