GBP to BTC via TransferWise

This is currently the cheapest way to buy bitcoins in the UK. 

TransferWise is a peer-to-peer money transfer service based in the United Kingdom, the service allows users to transfer money between countries and make foreign payments using the mid-market or interbank exchange rate.  

Any exchange that accepts SEPA deposits can be used but Bitstamp is recommended. Once you have a TransferWise and Bitstamp account ready follow the steps listed below. 

Create a TransferWise account http://transferwise.com/

1. Go to your TransferWise account and enter the amount in GBP you wish to send. You will then see the approximate amount you will get in EUR. 
2. Now switch to your Bitstamp account, go to the deposit section and select 'EU Bank (SEPA)'. The amount in EUR needs to be a whole number so just round it off. After clicking the 'Deposit' button you will be given Bitstamps bank details which you will need to complete the TransferWise payment. 
3. Go back to your TransferWise order and enter Bitstamps bank details for the 'Recipient'. It's important that you include the 11 digit number which Bitstamp provided. Check the details carefully and confirm your payment. 
4. Next you just need to choose your payment method. You can pay either by debit card or bank transfer. Bank transfers usually complete in minutes. 

TransferWise will convert your GBP to EUR (for free). Bitstamp will convert your EUR to USD (for free). You can then go ahead and buy your BTC.