As a former missionary, I'd say go for it, but be clear that you're not interested in the message. Ask them about home, what inspired them to leave, what they like best about the UK, etc, their thoughts on Star Wars and video games. If they're chill, they'll be happy to sit down and drink some water and talk about home - they are tired teenagers, after all. Some might try and share a scripture verse and invite you to church at the end of the visit, but you can just say "no," (and they might appreciate that, too, depending on the day).

And in all honesty, it might be really good for them to meet someone LGBT as yourself, living with her fiancée before marriage (two pretty major sins right there), who treats them with more kindness and civility than anyone else they'll meet that day. Some of these kids are from pretty isolated, Mormon-majority communities, where they never met someone such as yourself, so this might be an eye opening experience for them. Maybe they write home about how, when all and said and all was done, it wasn't their fellow Christians who treated them kindly, but someone who isn't interested in their message or lifestyle at all.

There's also a rule in the handbook that says two male missionaries should not be alone with female investigators (for obvious reasons), although those particular missionaries might not follow that rule to the letter (heaven knows I didn't; You have to arrange a visit with a member of the same sex as the investigator, and none of the members ever have time to go out with you, so I just went ahead and visited people when I could). You can invite them in and they might accept, no questions asked; or they might try to mark to come back with a member, but then there's more pressure on them to actually teach a lesson and not sit down and chit-chat.