Is it an OLCI printed boarding pass, a mobile boarding pass, one collected at the airport (from BA or collected from another airline earlier on the itinerary)?

As we've seen nothing like this before, a picture of it (with personal details including BAEC number blocked out) might help someone work out why this has happened.

Was it a flexible economy ticket (i.e. should have said 'Business UK' or just a cheap fare bucket?)

I have seen this before, but not in your situation. In my case it was at MAN checking in for a BA8xxx (ex-bmi) flight to LHR. You had separate desks for the former bmi flights and they still used bmi systems, so these systems thought C was business class and that's what was printed on the boarding passes. Of course these systems disappeared at the end of summer 2012.