I would have thought that your best bet was your home contents/buildings insurance provider.
You may have to pay a little extra to cover your laptop in view of its possible value, but it would still be more reasonable cost wise.
Incidentally, when researching such insurance recently (been with the Pru for more than 30 years but now it's farmed this area out to Churchill) I found that DirectLine, which I just switched to for my car insurance, was the cheapest by a long way.
You also get a discount for already being a policy holder.
I do not know if they still do it, but I think Norwich Union use to have a policy that covered computers under general conditions plus repair and breakdown. But if you want just 'theft only' then this should be available per your household contents insurance, if you have any.
Further to above, the Prudential has included (without additional premiums) home computer equipment cover in its house insurance for some years now.
I'm sure that other companies do as well but it will, obviously, be coverage for when you are away from home that you require.
Incidentally, when my two offspring were at university, we discovered by chance that the Pru would insure their personal possessions at their lodgings for the period they were away for a small additional premium.
A lot cheaper than students taking out cover themselves...:-)