Interesting question. Presumably if the are a UK citizen, then yes.
However given this mobs ability to craft shit laws, I can see people arresting tourists who have just turned 18 and carting them away for "Sunakification".
For the rest of the campaign, Labour are now bombproof. There is nothing any interviewer - not even Laura the K - can accuse them of that can't be simply rebutted with a "Yes, but let's talk about National Service, shall we ?" to take the curse of anything.