What section of a standard policy would cover this? I can see that I can cancel if I am delayed by more than 24hrs, but there is no section that seems to cover an alternate carrier if you booked carrier is unavailable. Would BA just pay up for this? I though they would just offer me a later date or refund. Are they liable to offer alternate carrier similar class travel?
There isn't and no they wouldn't. Yes they would. No they aren't.
This is standard BA forum practice at this point in the strike-threat cycle, to book a back-up ticket with another carrier. It makes a lot of sense (done it myself in the past) but you will be carrying the cost if you have to use it and asking BA for a refund of your original ticket.