What to do with an ISA

Herbalus

Open a LISA with £1 in it, because a LISA has to be open for 1 year before it is used to get the bonus for a house. Then if you do decide to buy in the UK, you can quickly lump £4K in it and get the bonus on at least that.

bitofsaving

I'd also consider how much you are likely to save going forward before taking money out of an ISA. You may not pay tax on any interest right now, but if you continue to save going forward you may get to the (happy) place where you have more savings than you can easily transfer back into an ISA under your annual allowance. The allowance is really high right now, but that's not guaranteed to stay the case.
For example, Leeds Building Society is offering a 1yr fixed ISA for 1.51% (not that different from Marcus), but you'd keep your savings under the ISA umbrella (if you transfer your existing ISA over). Or take their online access ISA for 1.38% - still a big improvement on the loyalty account.
You could still bank-hop with the new savings you put away going forward to collect switching bonuses and 5% linked savings accounts (which BTW usually only cover around 200/300 per month, so not that great for shifting over a larger lump sum).