Covering the cost of damage to your own car is only one part of the insurance premium. In fact it's a fairly small part.
A major part is to cover the risk of damage to other vehicles and the cost of injuring others. Even the tiniest bump can cost several hundred pounds to repair and if someone gets injured the PI costs run into thousands- and for bad injuries can be hundreds of thousands. Premiums reflect this as this risk exists irrespective of whether your own car is worth �20000 or just a few hundred quid.
Hence third party cover is no longer a cheap option- the cost can be nearly as much as comprehensive cover.
The premiums also have to cover the cost of MIB levies to cover uninsured drivers.
Premiums do seem high but when you see the massive claims that can result from the tiniest bump (car damage, whiplash, time off work etc) you realise why.