In reply to Simon Lee: Very good hypothetical question.
I think you are correct, we would need a number of graduations to properly describe the breadth of UK trad. 6 seems about right, although I'd interested to hear where you might slot in another 2.
Very well protected - G?
[e.g.The Foil f6c G - crack climb you can lace]
Adequately protected - A?
[e.g. Left Wall f6a+ A - safe but you climb above gear.]
Runout - R?
[e.g. Right Wall f6c/+ R - big fall potential, good gear]
Very Runout - R+?
[e.g. Rhapsody f8c/+ R+ - very big fall, ok gear]
Marginal/low protection only - X?
[e.g. Brown's Eliminate f5+ X, Mouse Trap f5+ X - crux marginally protected, rest of route completely unprotected.]
No worthwhile protection - X+?
[e.g. Hareless Heart f6b+ X+, Indian Face f7b+ X+ - unprotected.]
However, I'm not convinced this is a panacea. I would be more than happy for the Tech Grade of routes just to be supplemented on a route for route basis with sport OR BOULDERING grades, but only when and where appropriate.
In many cases standard UK grades are pretty spot on - Great Slab at E3 5b is far better than f5+ X. The same I think applies to most very bold routes up to E5 as sport grades don't reflect the style of climbing particularly well. Equally at the other end, grades like E1 5c generally wouldn't be improved with use of a sport grade and at that level, tech grades work equally well as bouldering grades. However when you get something like E4 6b, a description of E4 6b (V4) would be of more use to me than f6c G, which could equally be the same grade as a classic E4 like Arms Race which would definitely be best described as something like E4 5c (f6c).
The same thing applies at the top end - The Groove E10 (font8a+) and Rhapsody E11 (f8c/+) are probably the best concise descriptions available.