That's highly variable depending on the size of the transaction, how busy the network is at the time, and how long you are willing to wait. A typical transaction with 1 input and 2 outputs would be 144 bytes, but it could be significantly larger if you have to use multiple inputs. At the moment you could use a fee of 7 sat/byte and it would be confirmed reasonably quickly, so for a typical 144 byte transaction it would cost £0.23. However, if a retailer is using BitPay then they will charge you something ridiculous like £5 for them to move it to another address after they receive it. For a £1000 graphics card that wouldn't be a problem, but it would be if you made lots of smaller purchases.
