I’ve found very little difference, if any, between third party and fully comp quotes. You do tend to get more benefits with fully comp, such as windscreen cover.

They are politely letting you know that they would rather not insure your risk profile. This can occur because the insurer has underwritten all the risk that they wish to do so in a specific geographical area and do not wish to add to it further.

As a general point in mitigation of some of the comments, car insurance is a highly competetive business operating on very slim profit margins. This makes premiums very sensitive to weather events as well as inflation etc.