Heh, if there is one thing you
won't ever get from web designers, it's consensus!
Ask 2 designers, get 3 opinions.
IMHO it doesn't really matter in this case if you go for fixed or fluid width. Fluid layout is better for sites where readability matters: sites that people visit often, and that contain a lot of content. UKCTRA is not one of those, I think.
Fixed width can be a good choice if you want to do some fancy graphics, and need everything to be pixel perfect. OTOH, even with web standards and CSS-based layout you can make amazing pages; visit
CSS Zen Garden to get a taste.
I'd say you can design for a width of 1024 px. But allow for some whitespace at the sides -- this makes it look cleaner, and it also ensures that the content is still visible at smaller widths (900 px, or if possible 800).