I did learn that PostEurop, which is basically the European branch of the Universional Postal Union, is developing the successor of the CN22/CN23 forms at the moment. It is not ready yet and is one of the reasons why the EU did postpone the new rules. These new forms, which will also allow electronic data exchange besides the current paper data exchange, will allow packages with prepaid VAT.
PostEurop still lists Royal Mail as a member (which is to be expected, because the UPU is a United Nations body and the UK didn't leave the UN). Therefore it is IMO very plausible that Royal Mail is going to use the system developed by PostEurop, just like all other postal operators in Europe (and possibly worldwide). The conclusion is that there might currently be no way to send packages with prepaid VAT to the UK by mail. Combined with the new VAT rules this would be completely crazy
, but reading Royal Mail's website, I see nothing that contradicts this conclusion. (Not much confirmation either though.)
I read on DPD's Dutch Brexit page that they are going to offer a "Duty Prepaid UK" option, so it looks like at least one courier is going to offer a possibility with prepaid VAT. Other couriers may follow. I cannot check, because DPD is still refusing packages to the UK and the web interface for UK shipments is not yet available. So perhaps we should send items with prepaid VAT by courier? However, courier services are not the cheapest, and I haven't seen post-Brexit prices yet; this could be a disaster for low-cost retro computing items.