Route down: the short way.
Royston, Baldock, Hatfield, M25 J21A (which is a cut across saving 5 miles or so on the route via the A1M/M25 junction), M3, A303, Wincanton.
Total driving time: 3:30
Total distance: 150 miles
Route back: the route inspired by j4's Oxfnord <-> Cambridge route
Wincanton, A303, A34 past Newbury and Oxford where it becomes the A43, J9 - J10 on the M40, A43 past Silverstone, up to Northampton, past towards Kettering, back along the A14 to the Godmanchester turnoff and down the A1198 to Royston.
Total driving time: 3:15
Total distance: 200 miles
Admittedly, the roads were much clearer, but to take 15 minutes less on a route that's 50 miles longer?
(And it would have been both shorter and quicker if I'd checked the map and gone along the A45 between Northampton and the A14.)
Things which made a difference
Firstly, the A303 on the way down was suffering from pre-bank-holiday-Friday traffic, even at gone 9:00 pm. Coming back on the Sunday afternoon, it was fine. The roadworks on the M25 didn't help. But the worst bit was the impressive column of smoke emerging from a fire in the Hounslow region of West London, which was visible for a large section of the M25. I reckon to have lost 30 minutes just due to that, what with the slow traffic as everybody seemed to want to get a better look.
Normally, I'd reckon the short route at 2:30. In such conditions, the northern route doesn't make that much sense, since Royston is more than ten miles closer to the M25 than Cambridge is. But it also gave us the opportunity to drop in (at rather short notice - it would have been shorter if I hadn't missed the Silverstone turnoff from the A43) on my mother, who just happened to have my sister there too.
Conclusion - yeah, we may well use it again if we're not in a hurry to get home from western parts (anywhere to the west of the Oxfnord/Newbury line).