I think that it depends on where the holiday is flying from, when I had booked a trip LIN-LHR-XXX-LHR and wanted to upgrade the booking with Avios I had to call the "Europe" branch of BA holidays as the UK branch said that they could not do anything at all with the booking. This was a right pain as they never ever answered their phones and I spent 3 days calling whenever I got the chance until I got through. They then very quickly did it for me.

The Europe team is based in Bremen, Germany by the way.