I believe Macbooks use the same form factor as any other laptop, a 2.5" hard drive (compared to a 3.5" for desktop computers). Even in the UK, they should be marketed as 2.5" hard drives.
Exactly, there are apparently some thicker 2.5" drives (like the 320GB ones) that wouldn't fit in the MacBook or MacBook Pros. But anything lower in capacity than that should fit just fine.