I buy an annual policy usually worldwide excluding USA.
I use it for holidays Europe etc and adding on to work trips (usually Southern Africa)
I have never specified the dates i’m travelling in advance. The policy document just normally says trips cannot exceed 30days.
I got stuck in Malta few years back when it was snowing in the uk. I just put a claim in after and they paid out.
As long as it’s within the limits eg under 30 days and within the claim limit as specified on the schedule I don’t think they need to know when and where you’re going.
Edit....Sorry I misread your original question, blame it on being on the train.
Have to check the wording very carefully. Normally overstays for reasons outside your control would be covered.
Higher star rated policies will generally better and then the cheapest. Think the rating is called defaqto
Still best bet would be annual policy, there’s not usually much in it cost wise.