how old are you? under 25? thats the only time i've heard of a company not giving 3rd party for others cars.
because if you think about what comprehensive cover is - cover for your car (which is why you declare value mods etc) and others involved in accident - when you are driving a car that is not yours you will be covered 3rd party because they wont cover the car you are driving (if you are at fault) because it could be a ferrari, but the others because you will always pay for the other cars in a at fault accident whatever car they are.