It's part of program delivery and should be showing up as direct expenditure on their "cause".
exactly so.
furthermore, it is foolish of any charity to directly pressure (or more subtly have a culture of discouragement about) volunteers not to claim their expenses, or to suggest as people have in this thread that the charity will limit their costs to £X (eg half a shared twin room) and volunteers should shoulder the upgrade cost if they need better.
all this does is railroad volunteers into effectively making an additional 'donation' to the charity from their own funds. this (a) uses up the good will of the volunteers in a way that is really poor value for the charity - making volunteers reach burnout sooner after less value delivered and (b) means that these effective monetary donations to the charity don't get the advantage of Gift Aid.
they would be better off by far encouraging volunteers to claim all expenses in full, and be entirely open to booking individual hotel rooms etc, and invite volunteers to make any related donations explicitly and with a valid Gift Aid declaration.
eg a policy statement could read:
Expenses
Please do claim the full value of the expenses that you incur as part of your volunteering. It is important for our accounting to know the full cost of delivering the services that you are helping us to deliver. If you might otherwise consider not claiming your expenses as a way of giving a little extra financial support to the charity, please instead do make your claim, and make a separate donation with Gift Aid of the amount you would like to cover. Thank you very much.
similarly with the OP's situation they could have set it out like this:
Accommodation
we will of course cover the cost of your overnight stay as an intrinsic part of the cost of delivering the important activities you are training for. Our cost will be £55 per person for those willing to share a twin room with another volunteer or £95 per person for those taking a solo bedroom. Please indicate which you would prefer. If you would be in a position to pay for the upgrade between sharing and solo room options, this is more effective and supportive if you make the contribution via a separate donation to the charity - which can then include Gift Aid - rather than by shouldering any of these costs directly. We entirely trust your judgement in this matter and you are not obliged to make any particular choice.
wording the policies like this transform the attitude of the volunteers from slightly resentful discomfort to open hearted good will and benevolence. the good will of their volunteers is a massively valuable resource for a charity. they need to nurture and conserve it, not waste it for the sake of saving £20 here or there.