IME, it's usually more expensive to go for a pram in a bundle with loads of things including a car seat. You can sometimes find some good deals, but most of the time they throw in extras like a matching changing bag or footmuff which are £££££££ when you can just use a generic one, branded car seats and bases also bump up the cost. Unless the looks of the pram are very important to you, then it might be worth getting the matching bag.
It's worth getting the official raincover with your pram because they tend to fit better, and sometimes the proper footmuff is worth it, if there are specifically designed fixing points for example, but that isn't always the case.
I think the seats in your list which have features over and above the Britax seats are these:
Compact:
Avionaut Sky
Axkid One (all variants)
Suitable from newborn:
Avionaut Sky
Besafe Stretch B
Joie i-Prodigi
Axkid One+ / One2+
Self-tightening harness/self-adjust headrest:
Axkid Minikid 2, Minikid 4
Ability to change recline without fully reinstalling:
Besafe Stretch/Stretch B
Besafe Beyond
Joie i-Prodigi
Axkid One (all variants)
Isofix fitting:
Joie i-Prodigi
Axkid One (all variants)
Besafe Beyond
Spin:
Besafe Beyond
Honestly, I would probably discount Axkid Minikid 2 and Wolmax anyway because they are only suitable from approx 12 months which doesn't fit your plan of only using the infant carrier for the first 3-6 months.
I don't think the Klippan seats or Movekid offer much extra at UK prices.
You may also wish to discount Avionaut Sky if you're keen for it to last as long as possible because this one is a bit shorter than the others, although it will still last you to a good age to change to high back booster unless you have a very tall child. So is Besafe izi Plus (which is probably unavailable now anyway since it's older).
Avionaut Sky and Joie i-Prodigi also have a bit worse leg room for older children compared to the other models. The Britax seats only have leg room with the spacer block thing - I don't know if this is a problem.
Axkid One2 and One+2 are on offer at the moment so £500/575 depending on whether you want the newborn insert.
So based on your own cost rankings (I've added the £220 to the Beyond cost for the base)
£200 for a Britax seat (excellent safety, ERF, Plus tested, up to date)
-
£200 to also get a movable recline and better legroom (Besafe Stretch) OR a more compact fit + newborn suitability (Avionaut Sky)
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£250 to add isofix, a movable recline and suitability from newborn (Joie i-Prodigi)
-
£300 to add isofix, movable recline, compact fit, better legroom (Axkid One2) OR movable recline, better legroom, newborn suitability (Besafe Stretch B)
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~£400 to add isofix, movable recline, good leg room, AND either spin (Besafe Beyond) OR newborn suitability (Axkid One+2)
Sorry to add another thing to consider into the mix. Do you think it's likely you'll have more than one child? If so, it can actually make sense to buy car seats for all the different stages so you can hand them down, rather than needing to buy 2+ sets of each type.