Buying bitcoins in United Kingdom can be very hard... since Intersango closed doors and blockchain.info & MtGox stoped receiving GBP deposits.
We cannot directly send GBP to BS as they don't have a UK bank account. UK bank accounts appear to be difficult for some reason: look at blockchain.info who stopped allowing cash deposits at their Barclays account, and now disallow any kind of tras*fer in.
So in order to get money to BS, we first need to convert GBP into EUR. I'm going to do this with tras*ferWise (TW), primarily because they give much better rates.
What will happen is this: we deposit GBP into TW's UK bank account. Somebody wanting GBP in return for EUR does the same into TW's European account. TW will swap the two around: we can then use the other people's EUR, and the other people can use our GBP. For this, TW get a 0.48% cut. We then tell TW to send our EUR to BS, using SEPA.
BS use USD for trading, so our EUR needs to be converted first. Why do we convert GBP into EUR, and not directly into USD? Because EUR can be tras*ferred by SEPA for free, taking 1-3 days, whereas it would cost another 0.1% and take 2-5 days if we used USD directly.
BS accepts the EUR into our account, converting the EUR into USD. For this, BS get a 0.5% cut. Finally, we can use the USD at BS to buy BTC into our wallet there, then forward it from our wallet to wherever we like.