Yes there were no rules about singing outside.
No rules about groups sizes in schools. Even the SD was just recommended guidance with the understanding you couldn't keep those entitled to be in school and attending apart realistically on the account of age and emotional ability to adhere to it.
Rules for school children were totally different for those in a workplace or in private residence. The same as rules in shops differed and those for hospitals and care homes.
I find it worrying that people who think they have some superior intelligence to us and we just don't understand how Boris is right cannot tell the difference between singing indoors in a cabinet office with people who don't work there isn't completely different to school kids outside socially distanced and singing.
Especially as one was against the rules 