Significant caveat : not a tax professional. Take it or leave it!
Firstly, iirc you can only claim such stuff for a temporary workplace (less than 2 years). As your posting notice was 3 years this won't (
afaik) count as your belief was it was a permanent job. That changes at the point you were told it would only last less than 2 years. From that point on you should be able to claim temporary workplace relief. Look at HMRC's definition of temporary workplace carefully.
Are you talking about HTD or GYH? Either way you should be eligible for a refund of tax on mileage difference plus possibly your accommodation charges and food charges in the mess (not mess bills though) from the point at which it became a temporary workplace (when told of your tour lasting less than 2 years). All of this should be available to examine in fine details on HMRC'S website in the examples they give. It's fairly straightforward to understand and claim but read the detail of eligibility carefully.