The lack of Heathrow Express shouldn't be a big problem. For many people it's a toss-up between Heathrow Express and tube for that journey. With no HEX, it's decision made! (Piccadilly Line to Green Park, change for Victoria Line to Euston - I'm sure tube connoisseurs on here will pipe up if there is a better connection).

Didn't realise you were travelling over Christmas though. That makes things a bit more difficult in general, even before the strikes have struck.

The BA strikes should only affect 25 and 26 December, and even then only a small number of flights. They are prioritising keeping the long-haul flights running, with short-haul more likely to be disrupted (although hopefully not disastrously).

The Swissport (handling agent) strike is on 23 and 24. Depending on your airline and dates, you might not be affected at all. (Mind you, the fog or snow might still get you!)