I think they definitely do plan to expand, so we’ll see how they get on. Personally I think it could transform them into a global player.
I agree the US market has challenges, probably greatest among them being obtaining a banking license. However they can start that process soon and take years to do it with a dedicated team without distracting from expanding in the UK market, and the US is a huge market with IMO moribund systems. There are quite a lot of things about US banking that feel antiquated - use of cheques (or checks if you prefer), signing instead of chip and pin, no faster payments, monthly checking account fees. It is improving in all those areas but is behind Asia or Europe.
Interesting to note that both Barclays and Goldman Sachs have launched digital retail banks in the US recently (in the case of Barclays it is coming soon) - I don’t think they would do that if there was no demand, and there is a lot to improve about US banking.