Do you allow routes where one of the stations is a change/connection or are we purely talking about stations which are the start and the end of the journey? If the latter:
If my calculations are correct, there are over 3 million possible station pair journey combinations from Britain's 2,500 stations, IMO the number of those for which no one has ever bought a ticket would definitely be measured in 10s of thousands, possibly even 100s of thousands.
Firstly there's the numerous stations with tiny usage, even after accounting for enthusiast seeking out unusual journeys/tickers, I would expect the numbers of different stations from which people have ever bought a ticket to the likes of Sugarloaf, Pilning, IBM, Berney Arms, Redcar British Steel etc to be outnumbered by those stations from where no ticket has ever been sold.
The next big factor will be towns/cities with stations on separate lines but located within walking distance. Taking Wigan as one example, if you're planning a journey to Wigan where the most direct route involves either the entire journey or the final leg of the journey being on an Avanti or TPE service (ie the vast majority of destinations, probably well over 2,000 of the 2,500 British stations), there's no logical reason you'd buy a ticket to Wigan Wallgate. Glasgow is another example, from the majority of stations, there is only one logical choice out of Queen Street or Central to travel to, there's not many where choosing one over the other wouldn't result in a substantial increase in journey time.
Totalled up I think these two groups combined would easily account for 100s of thousands of theoretically possible station pairings, though in the second category we would need to subtract all the instances of people who booked them anyway due to either being an enthusiast who wanted an unusual routing plus regular passenger who simply didn't realise their mistake (and where the mistake wasn't picked up by either staff or online booking system at the time of purchase).
