Fine for third party but most of the cheap trade policies are fairly useless for the fully comp side of things.

And on the one I have you cant protect no claims bonus.

For example, if I had a M3 worth 5K retail and it got nicked, I reckon i'd get:
3500 quid trade price
minus 1000 quid excess
so 2500 quid.

That would lose me my no claims discount which is about 1K a year at the moment and would take a few years to build back up again.

So essentially it wouldnt be worth me claiming.

So IMHO on my policy a car would need to be worth more than 5K book price before it was worth even considering putting a claim in.