Unfortuntaley that is not correct. If you buy something from a UK shop that is shipped to the EU you will either not pay VAT but pay a net price to the shop and VAT is applied when the device gets delivered or picked up by you. Or the shop has a EU VAT regristration and you pay VAT directly on purchase which then is passed on to the EU by the shop (usually once a year or quarterly).
If you now buy a used module from the UK the seller most certainly has no VAT regristrtaion and you will have to pay VAT on import. The custom authorities do not care if anyone has already payed any taxes on the item in any country before and the price is now a net price. Though there might be the opportunity to claim an exception if you can prove that this very device has been imported to the UK when it still was under the authority of Europe and VAT has already been payed. But I don't think so as it would open a can of worms. I guess that with the segregation all british things count as exported now and are britisch again.
You can also apply for a VAT regristration if you are doing any business you get payed for. Then the VAT you are adding on your invoices will be substracted from the VAT you have payed on your (business related) purchases and you are allowed to pay net prices in shops who are ofering this like Schneidersladen. By the way that would also be the price Brits would pay now on european goods because they also have to pay VAT to their own sovereign now.
The Christmess deal is about trade and "real" business. It does not cover trading single used devices between private persons. Therefore it might have passages on how VAT regristations are handled but it does not cover your duty as a citizen to pay VAT to your state on all goods imported by you. Having all that centrally regulated was the main point of the EU and what made the british people feel uncomfortable.
On a side note this also applies when you fly to London for vacancies and come back wearing a tweet jacket that you did not wear before. It is just more difficult to control than it is when your tweat jacket comes in a parcel and DHL can inform the autorities that you will get somethine fromthe UK that probably is a good as it is sent in a parcel.
20 EUR is the threshold then for import VAT. Below that you don't have to pay it. There is also a threshold for custom duties. I think it is 120 EUR.
To sum it up the main point is that VAT is a tax you pay to your country for financing your infrastructure. They won't abandon it in favor of a foreign country.