Its a British thing
I think the problem is that the Apple UK keyboard is NOT like a UK PC keyboard with the predictable Apple-and-option-key changes.
Its more like a US keyboard with the Pound (hash) sign replaced with Pound (sterling).
This means that, compared with a UK PC keyboard the @, double-quote, bar, backslash and tilde symbols are jumbled up and the hash key is totally missing (you have to use option-3).
I've recently switched from a using a generic UK PC keyboard with my Mac to an Apple layout and have been bitten by this - under Parallels (XP, 2000, Linux) @ comes out as double-quote etc. Its worse with boot camp - the double-quote maps to some unicode symbol :-(
(The PC keyboard was proving an equal pain under OSX since, even if you install a corrected keyboard layout you can't change the default and some applications cause OSX to revert to AppleBritish).
There's a windows XP keyboard layout for AppleBritish available at
http://www.parkernet.com/applepro/ which fixes the problem for XP.
...but it would be useful if a future Parallels could add the AppleBritish re-mappings to the ctrl/option remappings so we didn't have to find a native solution for each guest OS.
I assume that the full layout/keycode details for the Apple UK layout are documented somewhere on the Apple dev site, but it may not be evident that the result is not the same as a UK PC keyboard.