Firstly, it’s my fault as I didn’t read/understand the full terms and conditions properly. But I’m sharing here in case it stops anyone else makes the same mistake.
The £10 deposit is good if you want to delay paying for the holiday but no good if you need to cancel. I found a lodge in a holiday park and booked for £10 to reserve it before checking I could get the week off work. Unfortunately someone else had the week off already but when I went to cancel the lodge they told me I need to pay the difference between the £10 deposit deal and the standard deposit (£193).
The T&Cs do explain this, though it could perhaps have been clearer. I made the wrong assumption that I would only lose the £10.
My fault but hoping I might save someone else from making the same mistake.