I can't speak for the UK but in the USA our laws technically apply no matter where a citizen is. US law trumps foreign laws and you could be prosecuted when you return.
No, they don't. One of the canons of statutory construction is a presumption against extraterritorial application of laws. Congress has to intend for a law to apply beyond US borders, and it's assumed they don't without evidence to the contrary.
In some cases they have done this, such as in the PROTECT Act of 2003 which contains a prohibition on child sex tourism. But there is no general assumption that US law applies everywhere.
And for good reason. Otherwise the entire Foreign Corrupt Practices Act would be unenforceable, as the bribery takes place overseas. Similarly, one could evade US law on insider trading or criminal conspiracies simply by moving overseas. If there's a nexus with the US, you can be prosecuted here.
So you say if I went to Amsterdam, where smoking marijuana is legal, and did smoke it there, and went back to the US, they could arrest me for drug possession because drug possession is illegal in the US and I possessed it while being in Amsterdam? Doesn't make much sense.
