I think you will find that the answer is there is no mainstream policy that will provide cover for such eventualities. You could no doubt get a policy if you went to a specialist broker, but it would be so expensive (as the risk of further strikes is very high) that it wouldn't be worth your while.
The most pragmatic option is to simply have a 'plan B' (e.g. renting a car) ready, and to make as many of your travel arrangements as possible refundable or at least changeable (e.g. Premier Inn offer the latter even on their cheapest rates).
Legally speaking, provided you book before a strike is announced (or possibly even after this, if there is no indication when booking that a strike might affect your journey), you are entitled to recover the reasonable cost of alternative arrangements. This is provifed the strike means there is no feasible rail option, and (as will inevitably be the case) the operator in question refuses to provide such arrangements themselves. However, you are highly likely to have to bring legal action to recover these costs - so you have to consider whether this will be worth your time and effort.