Taking this Horizon story as an example, the fact that we've rejoined shows that EU membership isn't necessary for it, but it still has "years out of the deal" as a negative for Brexit and I don't think any improvement to count as a positive?
No. The most you could claim is that we were a few weeks earlier with a vaccine than the rest of Europe, but then we'd have been developing our vaccine within an EU framework if we'd been in, so who knows what additional benefits that would have brought.
Exporting to the EU is hard: lots of standards checks.
Importing from the EU is easy: whole categories of imports are left unchecked, because new rules aren't being enforced as a matter of policy, which is the only way we have food security, for example.
As such it wouldn't surprise me at all if both your perspective and GP's perspective are true - cheaper imports would be better for everyone but the government doesn't "want" them because they'd politically inconvenience a relatively small number of people very visibly.
I was then surprised to see American cereal is now sold in the UK, and it's packed with the colourings and additives British parents in the 1990s would have refused to have in the house. Pre-Brexit, "No artificial colours or flavours" could be pretty much assumed for all cereal sold in Britain, even the colourful, sugary ones.
That's no longer the case. Example: https://www.tesco.com/groceries/en-GB/products/282819954
Contains Tartrazine (E102)*, Sunset Yellow (E110)*, Brilliant Blue (E133), Allura Red (E129)*
E102 — in the EU, not allowed in cereal
E110 — allowed, although the UK government previously asked manufacturers to stop using this
E133 — not easily determined from Wikipedia, although there's an example of British manufacturers stopping its use in the 2000s.
E129 — allowed I think.
They were commonplace in drinks aimed at kids (Tizer for example) for years.
Putting them in breakfast cereals is IMO mindblowingly irresponsible; the food producers know for sure (because the link has been established with little doubt) that tartrazine and sunset yellow worsen child behaviour.