Just to update with a completely unexpected happy outcome!
Our upload speeds remained frustrating for work purposes, and I heard Starlink is introducing 250GB/month data caps (or pay a lot more) in France which was very concerning. Locally on forums people are talking about Openreach rolling out to the local town now they've covered the villages first, and realising they were basically not yet finished in the area, I decided I had to be cheeky and go for the long shot, the last ditch attempt. So without a lot of hope, I emailed the CEO, explaining everything, asking if he could at least find out what the actual "engineering reasons" were for excluding us.
I felt like I was in a parallel dimension or something, because he replied five minutes later, passing my case over to a very helpful person who confirmed just as quickly all I had been saying. She said she'd get the local engineers to report back in a few days on what they could do.
Just a few hours under a week later, I am 100% not exaggerating, we have a shiny new box in our house with a nice bit of fibre optic cable on the other end of it. We're just waiting on the database stuff to come through (hoping this part does not take too long!) then we can order from an ISP and they can do a remote switch on.
I am pretty much speechless, and overwhelmed with gratitude at everybody involved at Openreach who took the time to listen, help, and get us FTTP and in such an insanely quick time frame. The engineers were lovely, very friendly and helpful, and worked super hard. They cleared out some existing ducts (as expected, our copper was buried, but they found some ducts they could use) and even brought the fibre into the house for us to save more time.
I feel quite guilty as I never asked and certainly never expected this level of treatment (realistically I was just hoping to find out why we couldn't get FTTP!) but from what I've since heard, it sounds like we should not have been left out of the rollout in the first place - they don't know why we were. It passed through here in about May I think, just before we moved, which probably explains why they were so keen to sort it out especially as they're still very much in-area right now. It seems I picked the right time to flag our property up.
Apparently there are four other properties that can now get it too who were missed out along with us, and at least two of those (our neighbour, and a farm over the road) came out to ask the engineers if they could get it now. So maybe this has bought us some brownie points locally as recently new residents!
Anyway, hopefully in however-long-it-takes-for-the-database-stuff-to-kick-in, we'll be sorted!
Meanwhile my entire world view about large corporations and response times for significant infrastructure work has been blown to pieces. Believe me, due in part to this forum, I know how incredibly lucky we are!