Try Direct Travel Insurance through Gocompare.com. I have just taken out a policy with them and from their policy document it states the following:
For Premier, Premier Plus, and Senior Citizens cover only;
f) You, your close relative or someone you are travelling with is a member of the Armed Forces, Police, Fire,
Medical or Ambulance Services or an employee of a Government Department and has their leave
withdrawn provided that the necessity to withdraw leave could not have been foreseen at the time of
applying for insurance or booking your trip;
Hope this helps.