If you are interested in ERF I'd defo recommend the besafe izi combi isofix, its brilliant and althought pricey I think it's well worth it! It's rock solid when fitted and pretty high so LO can have a look out of the windows. I've also heard good things about the joie stages seat which I am considering getting for out second car. Just make sure you get the newer model as the older model has a lower RF limit (18kg vs 13kg).
If you can't stretch to ERF then your best bet would be a combination seat like the britax first class plus (brilliant seat, we've got one in our second car but LO is close to the 29lb RF weight limit), you can normally pick one up for £80-£90 quid https://www.pramworld.co.uk/brand/britax/britax-first-class-plus?gclid=CJ3q04rT1rcCFVDJtAoddAwAmg Or the maxi cosi opal, both will keep you LO rear facing for a good while longer
I do agree with PP that the seat belt can sometimes be a pain but I just tuck it down out of the way, just remember to put it back up again! It's a gripe that I am willing to put up with to keep my LO RF.
I wouldn't even consider turning a baby of 5 months ish FF regardless of them being 20lbs of over. There is no way their neck muscles would be developed enough to support their head adequately in an impact, the thought makes me shiver
I've had whiplash from a low speed impact and it was horrible I can't imagine what that would do to a young baby