There is no legal requirement to have insurance for your tourer. When it's coupled to your car, the cars insurance provides third party cover, but it's wise to inform your insurer you are going to tow, most do not charge extra or impose additional clauses, but you should still advise them.
If you want to insure against loss and accidental damage then you need to consider a separate policy for your caravan. There are plenty of companies that offer policies, but you do need to check the details carefully, the cheapest is rarely the best. Only you can decide what is best, but some things to consider are:
New for old, at what age will the cover stop providing new replacements,
Storage limitations and security devices, where, what, and when they need to be applied.
If you want breakdown cover, check the policy does recover both car and caravan, and thatit covers your size of caravan.
Often with hobby activities like caravanning, there are some slightly unusual aspects to the insurance needs, and not all general insurers cover them adequately, as I suggested before you do need to check the policy details against your wants and needs. It's often the case that special interest groups like clubs have done a lot of research and have produced in conjunction with an insurer a package that is better tailored to the needs of their members, whilst they may not be the cheapest, the cover is usually good.
Another important consideration, is the companies claims procedures, how easy is it to access them, and are they rated highly by their customers for handling claims. Do your research before jumping into bed with your insurer.