I accept that I've never taught in a Catholic School, but have friends who send their children to such schools. What you have described here doesn't match what they tell me about the RE syllabuses that their children follow. OK, I'd agree that there are different syllabuses across the UK, (and not just within RE) and there may be some syllabuses that are more akin to the ones you describe than the ones I'm accustomed to.
By the way, how many RC schools are there in the UK, when compared with the total number?
Well my wife used to teach in catholic schools and I have close relatives, my nieces how have attended catholic schools in London and near to you, plus plenty of friends whose children do too. And my experience is that their RE curriculum accords very closely with that set out by the Catholic Education Service guidance (who of course are the overarching educational body for those schools).
So a good example of RE being to teach children to be catholics rather than about catholicism as one religion in an unbiased manner in the context of other religions being common curriculum in year 2. You may know this is the age when RCC expect children to take first communion (age 7 - way too young in my opinion). The whole year 2 curriculum is effectively an extended preparation for that first communion, specifically leading up to the ceremonies which are traditionally held in May. I've seen this in a number of RCC schools, not just one so it seems to be standard. Now nominally first holy communion and its preparation are extra curricular, but in reality this is run through the schools and through their RE curriculum.
The curriculum in these schools is astonishingly narrow, and about 'developing our faith' - in other words an assumption that the children are catholics and an objective to develop that faith further. To my mind this is really RI not RE.
And a further problem - who inspects RE in catholic (indeed all faith schools I believe) schools - not independent inspectors, for example Ofsted, but their own religious inspectors. So it isn't surprising that catholic RE inspectors inspecting a catholic RE curriculum aimed at developing young catholics don't kick up a fuss that the curriculum is narrow, biased and aimed at producing new catholics.
On numbers - in England (not sure about Wales) 10% of all schools, some 2000 are catholic, so this isn't a one off but a major part of the state funded schooling in England - and of course that is 10% of the funding for state schools as the funding per pupil for these schools is effectively identical to other schools.