I was sent some gym pants from a family member who lives in the US and they don't fit, is there anyway I can exchange them in the UK? Thank you.

if they sell the same ones they will exchange yes if they dont, i dont think they will phone them up and ask before going

You really should just contact the store/company in the UK, it's going to be hard to get an answer on here. Also, companies change rules so any answer may be based on the past, and if they tell you yes, it could now be no.