Hi there
It's hard to say whether you would be reallocated but if you firm Durham, there is more of a chance that you won't be reallocated than if you insure it. The reallocation priority order is listed here:
August Confirmation and Your College AllocationAlso if you rank self-catered colleges all higher up, it's easier for the allocation team to understand that you prefer self-catering and they'll try their best to allocate you to one. If you do get reallocated to catered, it's worth contacting your preliminary college in the first instance and explaining your situation to them, who can then signpost you to the allocations team if necessary. More info on this is here:
College Transfer RequestsAlternatively, you can look for private housing outside colleges (that's what I did too in my first year). I can signpost you to sources for searching for houses if this situation arises, but hopefully it doesn't. I'll keep my fingers crossed for you!

-Himieka