To come back to this,
I submitted an FOI to LNER to actually find out how their new first class on-board upgrades work.
Their network is now split into three zones, "London", Zone A and Zone B.
"London" is King's Cross and Stevenage.
Zone A covers Peterborough through to Lincoln/Doncaster/Hull.
Zone B covers the rest of the LNER network.
Prices range from £39 through to £69, and can be found on BR Fares.
If you're travelling from London/Stevenage, enter this as an origin. If you're travelling to them, enter them as a destination.
You can then enter ONTRN MDWK 1ST A (I007), ONTRN MDWK 1ST B (I008), ONTRN WKND 1ST A (I005) or ONTRN WKND 1ST B (I006) as the other location.
If not travelling to/from London, enter your boarding point as the origin, and the appropriate zone for your destination.
For example, a Zone B to London King's Cross on-board upgrade would be £69 at peak times, or £49 at off-peak times.
Purchase flexible single/return tickets & Travelcards – in 3 clicks. Full details of all British rail fares shown, with built-in Season Ticket Calculator.
www.brfares.com
I can't say I understand some of the logic versus the FOI PDF they released though, as fares data suggests Inverness to Peterborough (Zone A) is cheaper than Inverness to York (Zone B), which doesn't match what the PDF they have provided says unless there's some weird rules in the redacted issuing instructions.