This depends on how often you travel. If you do not travel often, then a single trip policy would be ideal. This does mean you will need to purchase a policy for each holiday or short weekend away you take.
However, if you plan to take 2 or more holidays or trips away, within a calendar year then an annual-multi-trip policy might be better suited to your needs, as this could save you money in the long run.
When it comes to single trip policies, it's best to purchase a policy as soon as you book, as this will ensure that cancellation cover is in place. Regarding annual multi-trip policies, it's best to start your policy, as soon as possible, as cancellation cover only comes into force from the policy start date.