Yes. You need to be completely upfront when you fill in an application form. If you fail to give the companies all the information that they are asking for, or you make a statement that is not correct, they will have grounds for cancelling your policy or, at best, demanding an additional premium. If they do in fact cancel the policy your chances of getting cover elsewhere will be severely reduced.
They will find out about it eventually anyway. Insurers share databases which hold information about practically every driver in Britain so they know far more about you than you think they do!