For most mileage possible with a ticketing-standard assumption about doubling back (i.e. not passing station A>station B>station A), but allowing the same station/group more than once with different routes, how about this?
Stage 1:
Wick - Inverness - Aberdeen - Dundee - Perth - Edinburgh - York - Hull - Doncaster - Derby - Nottingham - Grantham and/or Sleaford - Peterborough - Norwich - London Liverpool Street
Stage 2:
London Cannon Street*1 - Margate - Ashford - Brighton - Salisbury - Exeter (St Davids) - Bristol TM - Newport (S Wales) - Shrewsbury - Wrexham*2 - Chester - Crewe - Shrewsbury - Birmingham New St. - Leamington Spa - London Marylebone ,LUL central line to London Euston - Manchester (Picc) (via Stoke) - Liverpool Lime St. - Southport - Wigan Wallgate - Atherton - Salford Crescent - Bolton - Blackburn - Preston - Lancaster - Hellifield - Carlisle - Kilmarnock - Troon - Glasgow Central - Motherwell*3 - Greenfaulds - Linlithgow*4 - Glasgow Queen Street - Fort William, Mallaig.
*1 Alternative St. Pancras for HS1 if crossover acceptable.
*2 Additional mileage possible via Ffestiniog railway (if it counts) Shrewsbury - Porthmadog - Blaenau Ffestiniog - Llandudno Jcn - Shotton - Wrexham
*3 additional mileage possible if Glasgow Central - Newton - Motherwell
*4 depending on acceptable route around Falkirk.
Edit: could start from Thurso if Thurso > Georgemas > Wick > Georgemas is acceptable
Extra Edit: for Exeter to Bristol TM, possible Exeter - Westbury - Reading - Swindon - Bath - Bristol TM and several other detours are possible.
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(Long time lurker, but I think this is actually my first post...)
If anyone can do better (especially if you can keep the no-reversal condition!) I'd be very interested to know.
Of course, if you're going to permit reversal you can certainly get as far as, for example, Welsh's Bridge Junction outside Inverness, but to reverse you'll have to duplicate at least some track - so how much do you allow?
Have something start from Wick, take the avoiding line at Inverness, then Aberdeen, Stirling, Edinburgh, ECML etc... and run via the Kent Coast and Exeter to Fort William (or vice versa). Mallaig may count if a reversal at Fort William is acceptable.