the withdrawing of £15 is out of the question since that's the fee Binance charges, so if you plan on buying BTC worth £20 you end up with £5, so I'll say that's just as bad an as a hack.
You've missed from your calculation the fact that Binance's minimum allowed amount is 0.001 BTC (30 GBP)

I don't think the withdrawal fee of Bitcoin from binance exchange is that high like Ethereum right now, bitcoin withdrawal fee is still more cheaper than ETH, it's better and safer to withdraw your bitcoin to a private crypto wallet that you have control over
Just look at the fees from the link from my previous post. You'll be surprised.