By whom? I have not experienced this at all. STEM jobs are by UK standards well paid and would be considered solid respectable to enter by everyone I can think of.
Unless you're comparing the salaries in these to those of Bankers ... but that's a whole different kettle of fish, and there's the added complication that Bankers are actually actively disliked by an overwhelming majority of the population.
No actually real science jobs are paid appallingly I know what say the STEM grades in the civil service are paid.
And even developers are paid badly when compared to our peers in the states and with other professions in the UK.
Compare what google pays in London to what it pays in say Kansas I could buy a house there (Kansas) out if my cash savings
You do know that the train driver on my commute to London is paid £60k for a 4 day week and most of them after overtime are on >£75k with a DB pension!
The UK has been known for decades as a place to get good engineers cheap.
That's not a UK thing though - all STEM salaries in the EU are lower than their counterparts in the USA. I don't think this points to an anti-STEM culture specific to the UK.
But that is kind of beside the point - you said they're socially looked down upon. Maybe I derailed it by anticipating you were meaning salaries ... but it sounds like you believe there's some overall anti-STEM sentiment and I honestly cannot think of a time I saw anything like this.
I haven't heard of the idea that the UK is a place to get good cheap engineers. I have heard of Southern Europe, Eastern Europe and India talked about this way but never the UK. Good maybe, but certainly not cheap.
Or do you just mean they aren't considered attractive careers to students and children?