ISA 2018/19 oversubscribed - what happen now

HMRC will be made aware of the issue when all of the ISAs are reported to them and they'll advise you what will happen, which is likely to be based on the order in which the subscriptions were made, rather than you getting to pick and choose how you'd like to untangle it.

You can read up on it at:
https://www.gov.uk/guidance/close-void-or-repair-an-isa-if-youre-an-isa-manager#repair-void
Worked examples

DUTR wrote: »

My bank wouldn't allow me to over subscribe to my ISA in any fiscal year

It's a straightforward automated validation not to exceed £20K of new money into a single account, so to the best of my knowledge all providers implement that, but of course OP's issue only arises by virtue of exceeding the annual allowance cumulatively across multiple products.

DUTR wrote: »

IIRC when opening an ISA they check or you sign a disclaimer to say only one open per year.

The disclaimer would typically say subscribing into only one of each type in any given tax year as this is what the rule is. It's perfectly acceptable to contribute to four separate ISAs in the same year (cash, S&S, LISA, IFISA), and even five if multiple cash ISAs are with a provider that supports split ISAs (thereby effectively categorising them as one).

However, since OP paid £20K into a cash ISA as well as £240 into a HTB ISA then this would suggest that this is with two separate providers and therefore OP must have made a false declaration. While this was presumably done in ignorance it does demonstrate that it is possible to end up in this situation, and hence the question about how to get out of it....