Is there any difference between the two, meaning to provide someone with something they need or want? Which is used more often, and is it a British/ American difference ?
In UK we tend to use "hook up (with someONE)" for "meet (a person)", and also "hook someone up (with someONE" for "put someone in contact (with someone)".
We use "fix someone up (with someTHING)" when we help someone to get hold of something they want.
Both are very informal, mostly used by younger generations. "fix sby up (with sthg)" smacks a bit of illegal substances.
Examples: "Let's definitely hook up on Saturday!". "Did you manage to hook up with Sam when you were in London?" "Me and Ali hooked up when he came down". "I'll hook you up with an old pal who lives where you'll be staying". "I'll hook you guys up. I think you'll get on well."
"I can fix you up with one this afternoon". "Don't worry. I can fix you up".