Actually they hope the startup does well so they can buy them later. Revolut's cost of acquiring a user is far less than Barclays (<£10 vs £300+).
Barclay's deal isn't free, they take equity in exchange for letting startups using their infrastructure, it's not that expensive considering what they give though.
I think I know what you're getting at, at replacing the Correspondent bank system. There's another one, Transferwise, that seems to be getting transaction where Banks in other countries use it internally to avoid correspondent banks (transferwise often matches transfers to avoid transfers).
The BoE's 2020 system isn't for an ordinary firm to replace a banking partner. It's to join the payment network and have an account backed by the BoE/blockchain without it being at a designated high street bank. It wont be easy to use I guess, but it would be compelling enough for a Fintech company to use it.
I can see alot of value with the service you're describing, I don't think the level of value would be high out of countries which have high legislative requirements/forex controls. I had a similar idea a few years ago and my base assumptions were incorrect regarding people's needs for my service.
