Brighton is £559/mo for the Thameslink only ticket (the cheaper one) and 50.5 miles to London Bridge. Even being generous saying 22 days a month, that's 25p/mile.
Just give up.
I've proved your "A typical season ticket into London is under 20p a mile" claim wrong multiple times.
Southampton to Waterloo is just about the exception at ~19p/mile if you assume 22 days per month which I think is exceptional.
Annual tickets are 40 weeks meaning 5.75 trips per weekly ticket (230 a year). Newbury works out 21p a mile, Ipswich 23p a mile, Cambridge 22p/mile, so yes prices have crept up just above 20p/mile in the last year or two.
Alton to Waterloo is £5520 a year, 50 miles each way or 23,000 miles a year, 24p per mile.