I think there are a few things going on here.
One is that there are quite a few people on the left in the UK who are in fact OK with positive discrimination and argue for it, and often think it is legal like the US, and that muddies the waters. That might be fixed with just better education.
Another is that there are legitimate examples where you can have things like targeted programs. They need to show that this is proportional and legitimate, and it's important that can be done. So pregnancy related stuff might be an example that is obvious. But maybe another would be a local initiative to send out health info for women in a certain neighbourhood where there were a lot of women who did not have good English. Or funding a study on the educational experience of Traveller children.
However, I think people also feel that American style statistical bs is also being used, inappropriately, as a justification here. So - we see a gap in outcomes for this community, therefore it is legitimate to give them resources that we won't give to other people also in need. So, say rather than encouraging Traveller families to access a tutoring grant available to any poor family (or find out why they don't access such a grant) they have a grant only for Traveller families and other poor kids can't apply. This is a common type of initiative and justified on the grounds of special need, but quite a lot of people don't think it is really a correct use.
The thind issue is ino the most important, and it is also I think rather insidious. This is where being a protected group means that institutions feel a legal obligation to provide a service no matter what, and that ends up impacting others without the protected group status. So, for example, schools who go out of their way to keep kids with what is labelled a disability in the class, or give very large amounts of resource for things like rides to school. They do this because they know they can be in trouble if they don't support these kids who are from a protected group.
However, that can effectively end up with kids that are just "average" not getting supports they need. Like an orderly classroom. But there is no stick for the parents to pull up the school on this in the same way.
I think this problem is somewhat inherent in the practical effects of developing policy for protected groups. It shouldn't be, but it seems to leave a gap around others, I suppose maybe because the assumption is they get what support is needed. When there are limited resources to pass out, that isn't as simple.