It varies slightly. In England and Wales non-lead shot must be used for shooting ducks and geese (species specific) In Scotland and Northern Ireland the rules are that non-lead shot must be used over wetlands (area specific). Wetlands are basically rivers lakes and marshland. The differences are because of devolved government. Scotland, Wales and N. Ireland have their own parliament or legislature. Ironically England does not which means Members of Parliament from the other three U.K, countries can vote on English issues but English M.P.'s are forbidden to vote on issues which are only specific to the other three. Very frustrating when the Scots decide to scupper something that does not in any way affect them just to spite the English. That's politics I suppose. Slightly off the target but explains why there are differences in the regions. In all cases lead cannot be used below high water mark on the coast or on Sites of Special Scientific Interest or SSSI's.
Royalty are not exempt; except for the Queen and she doesn't shoot wildfowl as far as I know.
I know someone who uses a punt gun for wildfowling and by necessity loads his own. The only shot that will carry for punt gunning; other than lead, is Tungsten Polymer and he reckons it costs him 90 per shot.
This explains things far better than I can.
https://basc.org.uk/lead/ Lagopus.....
