If I'm reading this page correctly, to be legally sold in the UK a seat must conform to regulation EU R44/03 or R44/04 and must display a label to confirm this fact. It is legal to use a seat that doesn't conform to these more recent safety standards (04 being the most upto date), as long as the seat conforms to a British Standard or earlier version of R44 - however, I've found some of the other information on this site to be written in a way I disagree with so I'd check whether that is correct.
In general, I tend to refer any car seat questions I have to these ladies - I've contacted them a number of times over the 4+ years since DD1 was born and they've never failed to be incredibly helpful and extremely knowledgeable.
Going back to the original question, we bought a Recaro infant carrier when DD was born - which was great but quite narrow and by 7 months her shoulders were too wide. We then got a Britax 1st Class to keep her RF a little longer (which she hated) before discovering fully RF Grp 1 seats were legal in the UK - at which point we got a BeSafe IziKid, which she's just grown out of at just over 4.
Our first priority was very much what was safe in our car. Please make sure you buy a seat from a reputable store who will check the fit for you - my personal experience experience is that you're more likely to get a competent fitter at a small independent store than a multi-national chain that does not specialise in this field. If you have to buy on line, please get your seat checked - most county councils do free car seat checks at fire stations through the year, and a good retailer should be more than happy to check the fit for free. On line manufacturer fit-guides are a good starting point but they are known to be fallible, so please do check even if the manufacturer's website states it should all be fine.