You can simply ask what is and where you can find the company policy on business trips.
In this way you will probably be addressed to the document explaining the entire policy and you will get more information than the single question you have now. This will save you from keep asking questions over time, as it is normal to have them coming later.
If the "service owner" role was only ever positioned to you as a 12 month secondment then putting you back into your old role and pay is perfectly legitimate. However it sounds as though you're concerned that you are in effect going to be continuing to do the secondment role but without the associated position and pay, it sounds as though want to ...
There are a number of different ways companies handle this so talk to your Manager to find out the policy in your company.
The most common are:-
A daily allowance (per diem) to cover your stay.
Reimbursement of allowed expenses on production of receipts.
Provision of Company Credit card for charging expenses to
1 and 2 usually involve you paying out first, ...
Go and talk to a lawyer.
Citizens Advice Bureau might be a good starting point, but I suspect they will also tell you to visit a lawyer.
Absolutely do not return to work until you have consulted a lawyer. I believe there is a risk that if you return to work in the cold unheated warehouse this might be considered evidence that you voluntarily accept work in ...
Disclaimer. I am not a lawyer. If any of this sounds like legal advice, it is not. Any references I may make to UK laws are just based on my understanding. This answer is provided as guidance to help you make a decision on what to do next. If you decide the next step is to take any legal action, then I suggest you consult an employment lawyer first.
A lot ...