Which would make it GNU/NT…

Not just a U.K. company, the GPO were the General Post Office and ran the entire phone network and more: https://en.m.wikipedia.org/wiki/General_Post_Office

Happy to help! I wish I’d had as much foresight as you before buying my place…

Walk away NOW. If the issues are known ahead of purchase insurance won’t cover them. You do NOT want to have to declare subsidence on your insurance quotes - most brokers just won’t cover you. Almost everything you said is a huge red flag and there are probably more “nice surprises” to come. If you have a mortgage approved already then go and find another property.

If this is your dream house, at least get a building & contents insurance quote before you go any further. You don’t want any further surprises.

Edit: source: bought a money-pit house, thought we had subsidence, we didn’t but it was a huge hassle and a lot of stress regardless. We got a bonus moth infestation out of it, though!