G-BOAC and others are correct, my connection (which was nothing to do with BA) went tech. I am sure that award space was available to preserve my upgrade.
However, I am also sure that they went the extra mile because I spoke to a first agent who said nothing could be done as UK was closed and she had no training in miles bookings, but then said hang on, then got someone else who fixed everything ..... He also checked that my connecting flight was crock, and I had to be unchecked in from the BA flight, so I think there was an element of helpful rule-bending in play - hence my praise.