Most people will commute ~19 days a month, given vacations and sickness.

From my example, that's ~794 miles (corrected) a month, £255, £0.32/mile. Not as expensive as some other routes but still can be as as high as 12% of your take-home wage given the low salaries in that city.

Moving onto London... /under/ 20p a mile? Which route is that??

Just some random examples I picked:

Alton to Waterloo: £529, 1786 miles, £0.29/mile.

Guildford to Waterloo: £453/mo - 1140 miles a month, £0.39/mile.

Gravesend to London Bridge: £436/mo, 836 miles a month, £0.52/mile.

Brentwood to Liverpool St: £336/mo, 706 miles a month, £0.47/mile.

St Albans to Thameslink: £440/mo, 756 miles a month, £0.58/mile.

(Mileages from https://www.scotrail.co.uk/carbon-calculator )

Most people do 5 days a week, 46 weeks a year - with 6 weeks holidays.

Alton to Waterloo is £5520 a year, 50 miles each way or 23,000 miles a year, 24p per mile.