I reckon any old frame would do the job, there should be plenty of old British bikes available if you look around, an old Pashley, BSA, Raleigh etc town bike would do the trick, then just make or buy a rack for it.
I don't know what the post office did with all their old bikes now they've moved on to Pashley Postmasters, one of those would be ideal. One turned up on my local Freecycle group once but I missed out on it.
I reckon any old frame would do the job, there should be plenty of old British bikes available if you look around, an old Pashley, BSA, Raleigh etc town bike would do the trick, then just make or buy a rack for it.
I don't know what the post office did with all their old bikes now they've moved on to Pashley Postmasters, one of those would be ideal. One turned up on my local Freecycle group once but I missed out on it.