Legally they need enough tail to completely cover the vulva when adult. Breeds which traditionally winter on moorlands and or mountains need more, to keep the udder protected through winter.
I trim our lambs’ tails (within the first week of life) to part way down the thigh.
Keep them clean - wormed and if necessary dagged. They’ll be much more comfortable with a longer tail than too little.
And yes, it’s illegal and cruel to band them at this age. If it’s really a problem, you could ask the vet, who would probably refuse but would certainly use anaesthetic if they were to do anything.