As per promise to come back - I did track down a Whyte Cambridge, and it's a total dream. The weight difference is modest but still feels really significant - I'm very much aware of it when slogging up hills, or when picking it up to store it on the wall or for train journeys etc. Maybe the bigger factor is the carbon fork though - unless I'm just deluding myself, it's a massive difference from the Whitechapel: a couple of weeks since I got it, I'm still actively relishing the comparatively muted, gentle feel when rolling over rough surfaces. Not to get all grandiose, but it feels like the difference between liking and loving being on a bike.
The gearset is also a major plus: the gears both go lower (which I didn't realise I needed, but sort of did) and more smoothly. This does seem to come with more of a 'chainsaw' noise when freewheeling, but it's not crazy loud and I've grown to like this as a kind of less obnoxious bell.
All of which now also means I can start cherishing it for the long-term: I've got a dynamo hub/lights in the pipeline, which feels a bit like getting married.
Also, massive thankyou
@vickster for the Fb marketplace tip: I'd been scouring
Ebay/Gumtree, but pretty much as soon as Iooked on there I found a Cambridge going for a comparative song.
I only wish that everyone could hear the good word of this kind of bike!