Is it safe/allowed to adapt a shaver socket (BS4573) to a standard BS1363 plug (UK)?
No, providing an adapter to allow a standard 3-pin plug to be used on a shaver socket means that someone else may decide to plug in an iron or hair dryer or hair curlers which it is not rated for.

The shaver socket in the UK may be protected by a ELCB (earth leakage circuit breaker) which will also limit the permitted load. But in other countries that may be different, in some countries that shaver socket is just a normal outlet protected at the supply end.