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.
