Might be difficult. If you have already booked a holiday and the potential call up is known to you before arranging Insurance, Insurers will not cover a known risk.
What you should have done is to arrange Insurance at the same time as you booked the holiday. At that time, perhaps you were not aware of the potential call up ?
What you could do if you still want to look into this, is to find out which Insurers are forces friendly. There use to be a number of Insurers/brokers that provided particular policies that were more flexible i.e more understanding of forces personel and their partners/families. One broker link is below. You might find others if you search for them.