Most standard policies will cover you for towing a caravan or trailer. It’s also not a legal requirement to get separate cover to tow a horsebox or a boat, but it can be a good idea. Most important in any of these situations is to inform your car insurer of what you are planning to tow and see if it’s included in your 4×4 policy.
You’ll probably need insurance on a car you tow. The rule is if the wheels of the car being towed touch the road, it must be insured. This is true even if the car isn’t driveable. It could still be involved in an accident, so insurance is still required to cover that.