I leave my desk all the time... my cupboards aren't going to fill themselves with scotch and Toblerones!
But yes, there's no problem with working with people from other countries. Or people from the UK who are in different countries. The freelancing world is your oyster.
Don't worry about it!!
Just hire people who you want to hire.
If you're ordering a book on Amazon.com, do you need to know the country in which the author was living when he wrote the book?
No. Just order a book if you want to order a book.
On Upwork you are ordering business services.
You are not hiring an employee.
If an Upwork contractor needs to meet certain requirements in order to work in whatever place they're living, or if they need to pay taxes, or if they need to hire a plumber to fix a leaky sink in their house, that's all for them to worry about. Nobody is going to come to you and ask you to pay for their plumber's fees or ask you to pay their rent or pay for their Internet service fees or pay their taxes or whatnot.