Also interested in this as I have probably bought the last Sub (1961 1s) before brexit, I purchased it 29th Dec it was delivered 31st Dec, cutting it rather fine I know but very good service from Arendal. I wanted to avoid nasty surprises as had some larger than expected import fees for other stuff in the past.
Arendal will deduct the VAT from the price then you need to add Import Tax at 4% I think, then add UK VAT. And there may be a admin charge for the paperwork so figure about £30 for that.
What will be difficult to pinpoint is the conversion rate.
Wondering if a possible amp plate upgrade that could be purchased separately has been discussed? Not sure if that would be feasible for production, but would be a pretty cool option for current 1961 owners.