The thing is if you get another baby seat, as your DS is quite little you'll probably be able to use it for a while (maybe up to 15/18 months or longer) but most people find the baby seat starts to get cumbersome by about 9-10 months and the baby wants to sit up more and see out of the window. So you're really getting to the point that a lot of people prefer to change to the next stage seat anyway.
Obviously you do need to keep him rear facing. But the 7kg should be no issue as car seats which do both RF/FF are usually suitable from birth (or from 61cm)
There used to be a 9kg minimum for the next stage but this is outdated. You don't need to worry about this any more. Car seats are usually height based now and the threshold tends to be from 61cm which is about 6 months.
You could either show your insurance the receipt for when you originally bought your seat and base, or show them an up to date listing of the same model e.g. from John Lewis.
It is generally Halfords vouchers you get from insurance companies, they must have some kind of bulk deal. Halfords sell a lot of crap seats because they tend towards the budget end of the market, but they do have some good ones too. This is what I'd look at in your situation:
Maxi Cosi Mica Eco
Joie i-Prodigi
Cybex Sirona Gi i-size
Honestly, probably only those three. They don't currently have a Britax or Nuna seat.
Joie i-Spin Safe/Joie i-Spin 360 are also OK from a safety perspective but they aren't as nice quality wise as the Nuna seats and they have next to no leg room rear facing, which a lot of parents find tricky.
If you are totally set on getting a baby seat + base then get Cybex Cloud T i-size and the base for that, because it's the longest lasting infant carrier and it will last an average sized baby up to 2 years so if your LO is smaller you could use it longer than that if you want to.
But honestly it makes absolutely no sense if you are going to get the toddler seat to get a seat + base combo, when the seat + base attached versions are exactly the same seat (just permanently attached) and about £300 cheaper.
Unless you're definitely sure you want another baby in the future and want to use this as a chance to avoid spending out loads later on another premium baby seat + base combo? If this helps with that decision: You can get Maxi Cosi Cabriofix + base for approx £200 which goes onto the same pram adapters as the Nuna seats do.