There seem to be plenty about, though more often as a function of a convenience store than a thing in their own right now.
It'll be interesting to see what happens once the Royal Mail's mandatory use of Post Office as their sales channel expires (there was a time-bound requirement placed on the sale). After that they may well put their own convenience store arrangement in place anyway, leaving Post Office Counters Ltd (a nationalised business) to die off.
Edit: seems they already did renew it until 2032, but the Post Office is now allowed to sell other couriers' services as part of it.
The 11-year agreement will allow the Post Office to distribute parcels from third parties
www.thegrocer.co.uk