Like you, I'm trying to see what inter-Europe flights out of London might work for this promo. Iberia has somewhat cheaper fares to Madrid ex-London than BA, so getting a BA credit (for other than transAtlantic leg) would require a trip between London and Manchester, Glasgow, Edinburgh or similar city, as has already been noted.

Finnair's fares are a bit more pricy than IB, and the weather will be nicer at this time of year in Madrid or Barcelona rather than in Helsinki. But that's a city which is probably best seen in the winter.

All require Saturday overnight, of course. Good luck on finding something, and keep us informed.