There are no laws in the UK regarding "absinthe". In fact, there is not even a definition for absinthe in the UK. So a lot of drinks are labeled absinthe, but aren't actually absinthe. Just general laws that affect all alcohol, such as the maximum concentration of alcohol allowed (which is why imported absinthe from Spain is stronger).
This 3 shot rule people are talking about is a load of rubbish. It is just like purchasing any other drink in a bar. Perhaps they are experiencing managerial level rules, but not laws.
Also it is a highly overrated drink. Don't expect to be getting drunk off it in a shot or two like some people claim.