Last year (or 2006, I can't remember), when working out massive HST mileage trips, I totted up the following:
0400 SWA - RDG
07xx RDG - PNZ via BRI
1730 PNZ - SWI via BRI
23xx SWI - SWA
Did that myself in May 2004, some serious mileage covered. I believe I did notch up a higher mileage plan before, but I forget what it was. If I could find my FGW HST diagrams, I'd find you a high mileage plan off that. But I can't remember where they are.
Assuming, though, you're a fan of early starts/late finishes you could always do an early run from Ash Vale into London, get on the first limited-stop Pendo to Glasgow, come straight back and finish the same night in Ash Vale. 802 miles on 390s plus the other bits to/from Ash Vale.
If you're really hardcore on mileage building, you can't go far wrong with following this one:
0400 CDF - ABV
0509 ABV - PAD
0915 PAD - CDF
1155 CDF - PAD
1445 PAD - SWA
1829 SWA - PAD
2245 PAD - SWA
Going by the NRT, I measured that to be...1068.50 miles. Granted, it takes around 21 hours to do, but well worth doing if you really want mucho miles.
The other alternative is to do as others suggest and do YRK - KGX shuttles. Around 186 miles each way, so a few of those could deal with getting the magical thou in a day.
Oh, and congrats on getting so many miles in already this year Gizmogle. My best efforts thus far have gotten me 3,151.50 miles for the year.