The logical choice is something like MORTON BOATS BLOG: First boats for London Fire Brigade
Robust enough to take major abuse against jetties, nails, ladders etc without worrying about punctures. Easy to maintain. You could probably even drop one (engine off) over the side of a a harbour wall without worrying too much. More 'sea worthy' and comfortable than a similar sized SIB in my opinion.
Its baby brother the Secu12 might be OK too. Lighter to move on land, and a wee bit cheeper as a bonus. You might get away with roof topping one of them?
Other than being able to fold up a SIB (which is time consuming, risks damage and doesn't really solve the storage issue (it just lets you move the problem inland)) I see no advantage for your type of use.
Assuming the service is genuinely useful and welcomed by the PLA (or other bodies on the river) you'd have thought one of them would have provided some mooring space free or very cheap.