We now have a universal workaround for LNER fares.
For travel London <> all stations Newcastle-Edinburgh inclusive, DO NOT book to/from London:
1. Book to/from Finsbury Park
2. In the search results, click 'Filters' then 'Route options'. Under 'Go via ' enter Kings Cross.
Unfortunately, for travel from 1 October onwards, LNER are extending their 'fares trial', and discontinuing affordable/flexible/refundable Off-Peak fares on ALL their longer-distance flows.