I was wondering which sizeable town has the least direct rail route from London. I thought of Cheltenham, Gloucester, Hereford and maybe some of the big towns in the Pennines. If I'm not mistaken the shortest route from London to Cheltenham was via High Wycombe, Oxford, Kingham and Andoversford. But on the whole, since the GW cut-offs were built before 1914, most main lines from London have been reasonably direct, with nothing like the detour via Dijon that used to lengthen the route from Paris to Lyon before the LGV was built.
I am familiar with that most curious quirk of Cheltenham's, its lack of an obvious rail route to London. The Great Western Railway ended up all but given up on any hope of providing the town with a London connection with a velocity (as-the-crow-flies distance divided by time) of more than 30mph. In the 1949 Western Region timetable,
Table 15 details a multimodal option to Cheltenham: train to Oxford then Bristol Tramways bus. This was more direct but still an hour slower than the train via Kemble.
I have often benefited from this quirk with dubious ticketing, the frogging that it allowed me to get away with being widespread. Meek-natured Cross-Country staff would let me ride to Birmingham on a London return, then I'd bail at University to buy a suburban ticket to get me through the barriers at New Street, Birmingham local transport being relatively cheap. University had barriers but I always found them open, I suppose a backup plan would have been to go to Bournville or Duddeston but I never had to do that.
Any town in Northern Ireland.
Taking, for instance, Londonderry:
Waterside > Yorkgate > Larne > Cairnryan > Stranraer > Ayr > Kilmarnock > Carlisle > Euston makes up to be about 900 miles, or just under 50% deviation.
On a side point, I would myself quite like to use Steam Packet to travel from Ireland to Great Britain, using Douglas as an Emirates passenger might Dubai.
e.g. Belfast 1:30 > 6:01 Douglas 8:45 > 12:30 Heysham