I recall this issue before and its tied to currency. I believe that the BA Holidays system cannot cope with a change of start point if it involves a new country and currency and from the sound of it you would be swapping EUR to GBP and this is more likely the issue than anything else.

I have seen others have it refused as it involved currency changes so perhaps this is the issue. I seen one case in particular where they wanted to swap from AGP to GIB and were refucsed as the fare was in EUR and Gibraltar was in GIP

Mainline BA have no issue with this but the BA Holidays system specifically does,