If you don't have a friend with bitcoin who is willing to sell them to you, the best option is to buy bitcoin off an exchange.
Lucky for you, being based in the UK means that you can use Intersango, the largest GBP/Bitcoin exchange in the world.
Assuming you have online banking, once you have set up an account on Intersango.com
on average it takes an hour to make a payment to your account (providing your bank supports fasterpayments and processes the payment quickly).
As well as being the largest GBP/Bitcoin market we also have the best customer service.
If you have any questions on ANYTHING just email
support@intersango.com