Speed of light is an unknown number unless you name the medium it is travelling in I did.

Ah, fair enough. I suspect most (including me) wouldn't have picked up on that.

Replying to self to finish this matter off...

First, I've pretty much confirmed that my connection comes out in London by using Ookla Speedtest's manual server selection feature. It autoselects servers near Cumbria based (I assume) on my geoip, but as I force servers closer to London, the ping goes down. Servers in London itself give ~11ms RTT, which seems to be the absolute limit for me.

Second, I fired up a London DigitalOcean VPS and tested latency to various servers in the north of England (no servers in the south of Scotland, but close enough. Used speedtest-cli since it was the easiest way to ping particular locations). Those came out as around 11ms too, so it looks like this time is quite normal.

I'm sure sufficiently determined parties could improve this somewhat by using longer reaches of dedicated fibre/better routing/faster routers, since it isn't at the physical speed-of-light-in-fibre limit yet. But it's reassured me that there doesn't seem to be anything wrong with my connection.

A PPPoE connection that ended earlier than London would allow lower latency to servers north of London, but in practice most of my traffic would likely go through there anyway, so it works out well enough.

Thanks all for the info and advice

If you have a need to emerge at a location closer to you, consider some of the smaller providers who may have their core in Manchester