@anna38 , going through all of the above, I suspect that based on the bank's understanding of your situation ( and probably not being aware of the Green Card having been issued), there was a "presumption" that you have become a "Non-US person " and hence the issuance of the 1042-S. The facts and circumstances ( based on above ) suggest otherwise but that is of no help in your situation.
1. So , absent a statement by you to the US authorities that you have abandoned the residency ( or violating the required visit to the US rules) , you are treated as a resident alien.
2. The above means that you file a form 1040 and be taxed by the USA on your world income henceforth
3. This also means that you essentially have to create an alternate/dummy 1099-R for withdrawing your pension/401(k) etc. , pay the usual tax AND also the 10% penalty for early withdrawal ( I think this is suspended for 2020 because of the pandemic -- I/you have to look this up ).
4. Even if you have absolutely decided to NOT ever be a resident of the USA, you should visit the US Embassy/Consulate in London and clarify your options regarding this.
Is there more I can do for you ?
Cheers
