is a pub or club breaking the law by not having any notices of the under 21 policy in view of customers. or dont they have to have these notices at all.
Assuming that you're in the UK (where the minimum age for purchasing alcohol is 18), rather than in, say, the USA (where the age limit is 21), the 'Challenge 21' initiative is purely voluntary; it's not required by law. (As long as the bar ensures that they don't serve to under-18s, they're operating legally; how they do it is up to them).