Many thanks for the input and suggerstions to all of you!
My problem is that so far I was unable to find a shipping/customs agent that is willing to do this (or even help me with it).
I would even pay both VATs if I have to, but I just so far couldn't find enough info on how to proceed and what I will have to do customs-wise if I self-pickup in the UK.
I would have thought that I simply declare the chassis at customs when crossing borders, using the invoice as proof, but this does not seem how it works.
I've looked into the Shielders option, but they don't offer the extras that I want (like 2nd underseat tank - to be honest I've only checked their webpage so far) and my gut feeling is that despite some slightly negative info on Richards recently here and elsewhere, they are still the place to go.
And I have thought about having it changed in the UK (obviously changing it myself is not an option for lack of space, time and logistics even though such a 'holiday' as Oilstain suggested would probably my best holiday ever). But for me the chassis swap is a once in a lifetime thing, I guess, and the journey is the goal, so I want to do this properly (painting chassis, proper nuts and bolts, loom, ...) in my shed for as long as it takes. Ideally I will remove and fit 1:1 with both old and new chassis next to each other (also to avoid problems due to bad memory), so a quick 'bodge' job will just cause later frustration, I guess.
I will ask Richards again (may also be my lack of English) and see if they can advise further. They wrote that
"it would be delivered to the required address but you would have to sort customs, taxes etc.".
And that is the thing. I don't know and can't get info on how I am to do this.
Self-pickup would also be ideal as I could get other things I need and carry them home myself. The mailorder option is horribly expensive post-Brexit with not only customs but also a 'handling' fee for any purchase by the courier. Good old times when I could just buy an oil filter element at Paddocks and some other consumables and have it at my door for a fiver ...