There's a thread in Car Seats right now which is talking about this very topic. Which would also be a more appropriate place for this post, TBH.
You can look at the Which? reports in your local library, or pay to view them online.
Halfords won't be able to tell you of course, because there is no objective measure of "the safest". The Which? tests will tell you how they performed under their specific safety testing, and of course any seat on sale will have to have been tested to legal standards. Obviously that's a minimum and some will have higher protection than others. But there is no NCAP-style rating.
I have a Britax Adventure and find it great but Which only scored it 42%. Personally I think that you can overthink car seats. The most important thing is that they are in one which is appropriate for their size and weight. I wouldn't use nania seats, but I'm happy with the Adventure.
HBBs aren't very heavy, TBH I wouldn't have thought it would make much difference. ISOfix stops the seat from coming out of place or flying around in the event of an accident where the child isn't in the seat, with a non isofix seat you should always strap the seat in even when it's empty.
If you want to keep them as safe as possible, then you could always buy extended rear facing, 5-point-harness seats, there are some which go up to 25kg.