Heres something that my sister just posted on Facebook:
Question from kaylascousin for anyone in the UK:
I’m doing some research on flag etiquette across the pond and I was wondering if it was allowed that three flags may be flown on the same halyard, e.g., Union Jack, Flag of Wales, and a School Flag from top to bottom.
I feel it incumbent upon me to stress that this is HIGHLY unlikely to be a request for assistance with homework; this particular cousin graduated high school five years ago, and is currently teaching classes at a Portland (OR) area art school.
This appears to be the official answer, which suggests that what your sister outlines is OK in the UK. I have a vague recollection of doing something similar at my school boathouse decades ago.
What a dedicated Welsh nationalist might say about it, I don’t know, but I’ve never heard of a non-governmental body or individual getting into official/legal trouble over this sort of thing.
When you say halyard, do you mean on a flagpole? I guess you must because otherwise it would be on a ship. It makes a difference because most of the regulations in the UK are about flagpoles.
I’ve never heard of a school flag in the UK. A lot of schools have emblems and I suppose it’s possible they could put them on a flag some time. Eton appears to have house flags (for the individual houses within the school) that they use for competitions sometimes but they’re carried around rather than mounted.
If it’s actually a “school flag,” whatever that is, you probably don’t need planning permission (from the local authority) too, because it would likely count as a “house flag.” As long as it meets the other regulations, that is.
You can also only fly two flags at once, within the grounds of one building, without planning permission. That doesn’t mean you can’t do it - I know I’ve seen way more than two during some sporting events - but you’d need permission to do so and it might well be temporary. When I’ve seen multiple flagpoles they tend to be at places like the Olympic Park or somewhere that’s actually owned by the council, not private property.
I think the problem with flying three flags on one flagpole would be that, for all three flags to be visible and not obstructive, the flagpole would have to be tall enough that it would be against the rules for that reason. Again possibly you could get planning permission for it but it’s not a standard thing to do.