I recently picked up a second hand steel spaceframe which came with crests, 2.35 Nobby nic rear, 2.35 Magic Mary front, and a spare front 29er Hugo with a Knard.
The rear had a bit of a wobble in it and I was keen to try bigger, so I put an Arc 27 (internal) on the back with the same nobby nic and the knard on the front.
I prefer it like that, but the Knard is outside it’s ideal conditions window. OK most of the time, but sketchy on steep, muddy switchbacks, as any fule kno. Occasionally I google Dirt Wizards then baulk at the price.
I’d put the MaMary back on the front, but that would have to be on the crest. It would be weird to have a wider rear rim than front, so it’s staying as it is for the minute.
I picked up some cheap 650 Arc 45s, but they’re 28h. Got a rear 28h hub, but there’s no-one that does a 28h 135mm front hub. Good to know that 29+ front, 650b rear is a decent option.
If the Chilterns are too far for you, I’m guessing Surrey/Sussex border is as well? The offer’s there though, as per Jameso.