Wow. I didn’t realise that. That sounds like a major headache. Fingers crossed for you and your industry that our government gets its fingers out of its arse and sorts things out for you.
The only "sorting out" the government can do, is to get the new GB regulations drafted and published so that we know what we're working with. The rest of it, I'm afraid, can't be sorted. That is just the nature of Brexit, I'm afraid. We voted to make ourselves a third country and now every car that we sell into the EU will have to jump through the same hoops as (say) an American one would.
When I started my career in the late 1980s, every EU country had its own national type approval regulations. Selling across Europe was an absolute pain in the arse! French cars needed those silly yellow headlights, German ones needed parking lights, Italian ones needed a horn that made a particular note (I kid you not). Different exhaust systems for different countries with different noise limits... Absolute nightmare! Then in the early '90s, we (the countries of the EU) finally got our act together and a greed a common set of requirements which, if met, would guarantee unlimited sales in any Member State, without any further tests or red tape. It was paradise for us! Instead of two-dozen sets of similar but infuriatingly different sets of paperwork (and maintaining them as each country amended or updated their regulations), we had ONE! Better still, we got a say in the content of those regulations!
As a result (partially) of this reduction in red tape, car prices came down. (I say "partially", because it was also down to the EU "Block Exemption" regulations that prevented mainland EU manufacturers from ripping British customers off, charging a lot more than they did for the same car on the continent).
I've had Brexiteers say "yeah, but so what? Car manufacturers still sold across Europe before - we can just go back to doing that". However, what they fail to realise, is that back then, the requirements for type approval were a LOT simpler. About 2 dozen separate tests compared to nearer 100 now... Of course, having "taken back control" we can, of course, go back to what we had in the '80s, but there are a couple of problems with that.
Firstly, British consumers would have to accept safety and emissions standards like we had in the '80s;
Second, what keeps most of the cheap tat from China and India out of our country right now, are the tough EU type approval requirements. Remove them, and we'll be knee-deep in "3rd World" cars that British manufacturers simply won't be able to compete with...