There’s no magic way to make this trip make sense without a lot of backtracking, because you have selected places that are widely spaced across the UK - while missing out many great areas in between.
A few thoughts: Brighton isn’t really all that, and if you do want to do it, best as a day trip by train from London (are you spending some time in London as part of the trip?)
The Cotswolds (not Cotswold, singular) is a very large area, not a single place to visit.
The Lost Gardens of Heligan are way down in Cornwall, a looong drive at the best of times. And if your trip happens to coincide with the late May school holiday week then the traffic can be extremely heavy. Also, to get to Cornwall, you would be driving through gorgeous Dorset and Devon, full of stunning scenery and lovely villages, and missing it all because you’d probably be sticking to the faster roads. Would you, for example, consider dropping the overrated Cotswolds in favour of Dorset? Similarly full of cute villages but somehow there’s this idea amongst overseas tourists that the Cotswolds is the only place like it in the UK.
Alternatively find another of the UK’s gazillions of lovely gardens that make more sense with the rest of your trip - such as in the Cotswolds: Hidcote or Snowshill for example.
I don’t know Wales at all.
York is lovely and well worth it. Although there are many other lovely cathedral cities in England.
You might take a train from London to York, sightsee there then pick up a rental car, do your North Wales and Cotswolds thing. Those could work in a big circle.
Cornwall is an absolute outlier - unless you replace the Cotswolds with Dorset and Devon, and choose somewhere like Salisbury or Winchester as your cathedral city instead of York. Or maybe lovely Oxford.
We can’t magically change UK geography but I can assure you that for everything on your list, there are alternative areas that are just as lovely and might make more geographical sense.