Is that limit applicable domestically - like would children be able to enforce it against a carer/parent. Or does it only apply to commercial arrangements?
Do you get to demand 20degC at work?
20degC is a bit hot for my liking, 18 degC as a minimum, perhaps - one should expect to wear a jumper in Winter.
Yes, if it's office work or other work without physical activity. Limits are lower for more active work. (and of course special rules exist for environments where working in cooler/cooled rooms is required)