I'm not sure. What I can say is that bank transfers seem to be a viable way to pay for BTC in the UK. I use LocalBitcoins (in the UK) for purchasing BTC, and the quickest way to pay seems to be "National Bank Transfer" - usually a two-hour (max) wait for the funds to transfer, then LocalBitcoin release the BTC from escrow.

I gather it is not this straightforward in the US? (And, to be fair, this is all a rather recent innovation in the UK - two hours seems mind-blowingly fast after the three-five business days we've had to tolerate for most of UK banking history!)

Having said all that - for a UK-UK transfer of your hypothetical $400 - if both parties have UK bank accounts, the cheapest and probably quickest way would be a bank transfer - no fees, and the recipient should have their money in a couple of hours. (Assuming your $400 had been converted into GBP - if not, then add on FX charges etc). Anything more complicated than a UK-UK transfer, however, and I'd look to use BTC.