The salary isn't that out of line for a mid-level developer nationwide, but yes I would expect it to be higher for the southern location. They could justify the salary if it was fully remote.

Bear in mind there is a 23.7% pension contribution from the employer, so it's a roughly £62-70k total comp for a mid-level role.

Edit: Actually though, in reality I would expect a salary bump to work in the public sector to encourage one to put up with the terrible working enviornment with all the bureaucracy.