You can still do it electronically, it'll just take more than a few minutes as it has to run through the bank's daily reconciliation process. That's probably OK though, I doubt many people have the need to transfer > £250,000 and the entirety to be available immediately.
In the US, anyway, I had to do such big transfers via wire transfer, and while it happens (almost) immediately, you have to catch the 10am or 11am (Pacific) deadline, depending on the bank, or else you're waiting another whole day.
