The Swedish style ERF seats are designed for tall, leggy, Swedish children! And the European regulation is accepted in the UK so you can legally use such seats here.
They are usually designed so that they can be set back a little bit from the back seat, which allows space for legs as though they are sitting in a chair, but even if they don't have this space due to the seat design or it being fitted in a more compact way, then it's not a problem at all for children to cross their legs or have them resting up on the back seat.
I am looking at my 2yo right now who is sat on the floor watching TV with his legs in a complete W-shape. And my 5yo is snuggled up under a blanket with his legs pulled up to him, too. Children of the age to be in a car seat are not generally choosing to sit in a position like they would in a chair in a classroom. They are much happier than adults to sit all scrunched up.
People come up with all sorts of objections to rear facing but they are almost always not founded in reality, and are mostly about the person's own familiarity with forward facing - we tend to be more comfortable with things that seem familiar to us and less comfortable with/more sceptical about things which are outside our previous experience. So there is a certain amount of "looking for" problems with rear facing rather than looking objectively at the two options. For example people make an assumption that a RF seat will take up more space or be more difficult to use, without actually looking at the seats and comparing with a FF seat.
That said, I am not as staunch an ERF advocate as some in the car seat world. I'm a strong believer in if it works for you then absolutely keep doing it, as the safety benefits are well worth it, I also think it's well worth promoting overall as a "standard" option, and I do support lobbying industry to offer more ERF options which solve some of the problems people have with RF, and I will happily help any individual find a RF seat which meets their needs if I can. But on an individual level - if it does become a hassle at some point, for example due to car sickness (which not all children suffer from), budget, space, or any other practical reason, where a seat which would solve the problem is not available or accessible, if the child is over two, and certainly the older they are, the difference isn't so great that it justifies a lot of inconvenience. I also think that a strong focus on RF to the exclusion of any FF at all causes an issue in that parents find it difficult to find solid advice about FF seats, how to use/fit them correctly, what is important and the differences between them.
In terms of policy, I think the current law is fine, but I would extend the 15 month rule to all seats (not just R129 seats) and increase messaging emphasising the preference for rear facing seats for the whole of the "integrated restraint" (where you use car seat straps rather than the car's seatbelt) stage.
Basically, if you're in a crash then there is a chance that the crash is so extreme that no car seat will save them. There is a chance that the crash is so minor, even an unrestrained child will be fine. And then there is a large area in the middle where it matters what seat they are in. This area in the middle is rather larger than most people assume. Most people think that either a crash will be minor and so children will be fine whatever, or a crash will be severe and then nobody will be fine. The truth is more complicated and children are much more vulnerable to injury in a car accident than adults.
Unrestrained > restrained is the absolute biggest difference, except for children under the age of 1, where a forward facing or booster seat (yes, it happens) or a seatbelt alone is almost as poor for outcomes as being unrestrained.
For children over the age of 2-3 years old, even a seatbelt is MUCH better than nothing.
Then you have improvements in outcome over and above "any restraint" for the in between area, for each type of seat - a booster seat is better than just a seatbelt, a 5 point harness seat is better than a booster seat, a rear facing seat is best of all.
The second biggest factor in whether or not the car seat will improve outcomes is about how it's installed and used - there are some really common misuses which still make a bigger difference than forward vs rear facing.
So rear facing vs forward facing is actually the third most important factor (after using anything at all, and ensuring it's used properly). However, it is the most important/significant factor in the seat choice itself.
Other aspects like side impact protection, isofix vs seatbelt, old regulation vs newer regulation, construction material, individual safety features of a seat etc are less important. These are all negligible compared to the other three factors. Arguably the most important aspect of a seat's design is how easy it is to use correctly and avoid misuse.
So: A cheap/basic/"bad brand" or old fashioned RF seat will offer better protection than the best, highest performing FF seat (possible exception: Cybex Anoris).
BUT misuse trumps direction. So even a cheap/basic/"bad" FF seat used properly is safer than a RF seat which is being used incorrectly, if the misuse is severe.