You claim subsistence or vouched. Once you go down one route that's it, you can't chop and change to suit yourself.
The agreed method should be clearly stated in your employment contract.
If you decide to stay in a tent, that's your own business!! You get the agreed rate once you demonstrate period of absence not that costs were incurred. Remember the subsistence rate is to cover accommodation, food, etc so you are not making a packet on it.
The point of the civil service rates is that they can't exceed this amount or the whole amount becomes taxable.