I know in the US we have personal article policies that allow you to insure specific items. Most of the time they're used for jewelry, but I have seen them for electronics.
^ You can insure your property even when renting a home with others. The policy as a whole shouldn't cost much and scheduling items such as the iMac is a good idea.
I can't say what the cost will be in the UK (I imagine it varies within the UK as well) but my renter's insurance here in Toronto Canada, where the insurance for everything is the highest in the country is only $25 a month.