Um. No.
The vaccine was approved for use in the UK *before* the transition period ended, *because* of EU law.
Also, you're paying a lot more per dose than anyone in the EU and I don't know why, because the UK was invited to be in the group order *without* having to abandon brexit but just didn't respond.
Also also, if you can get all you require, why on earth are you talking about skipping the second shot or substituting a different vaccine for that second shot, to which every doctor involved has said "are you high? of course that's not how that works, can I talk to an adult please?" when asked if it might work.
Yup, a lot of shops here are pulling that nonsense and paying for it. Our kettle sprang a leak and shorted out the main house RCD. Ordered a replacement from the Curry's down the road (we're under lockdown with the worst infection rate per capita in the world right now thanks to nonsense from our bunch of merry idiots). That shop said it had loads of stock, all would be well, so I ordered and they tried to ship the kettle from a depot in the UK via DPD who had just announced that they would indefinitely suspend shipping from the UK to the EU including Ireland because of Brexit. I cancelled the order, and apparently I'm one of thousands doing the same thing, which can't be great for their business.
Meanwhile the same freight issues biting in the UK right now are also biting here, even though we never had a vote on whether or not to have a go at this jolly jape, and our freight companies are giving out the most dreary warnings about foodstuffs and such, which is irksome when you're under lockdown conditions. So, y'know, brexit is ever so popular over here right now.