French Foreign Legion

Hi guys

I'm still waiting for my decision for naturalisation and I was just wondering about my past when back in Feb 2016 I went to France and wanted to join the French Foreign Legion... but didn't pass the selection process... spent there about 25 days, came back to England on 28th Feb 2016, made my application on 05-March 2021 - will this be a problem?

I haven't declared anything but is it illegal to join the foreighn forces in British law?

Doubt the UK has any law prohibiting UK or foreign residents joining foreign armed forces. There's nothing in the "Good Character" guidance about this. And in any case you never joined the legion. There's nothing to declare as far as I can see.
I do know that there are UK citizens who serve in the US armed forces and indeed there are Commonwealth citizens serving in the UK forces (notably the Gurkhas from Nepal).