If you are not sure which type of policy to choose, here is a summary to help you decide.

Single Trip

A Single trip policy covers you for one holiday of up to 30 days. Cancellation cover begins as soon as you have bought. All other cover begins when you start your trip. All cover ends when you return from holiday. If your return is delayed, don’t worry, you will be covered until you can get back home.
To get a single trip quote, simply choose the number of days you are away, or click on the calendar and select the dates you leave and return. Remember that if your holiday is 7 nights, you need 8 days cover, and if you are away for 14 nights, you need 15 days cover. 

Annual Multi-trip

An Annual multi-trip policy can save you money if you are going abroad more than once this year. It offers cover for up to 90 days abroad in the year, but no holiday can be longer than 30 days. If you take the Winter Sports Extension, this allows cover for Winter Sports abroad for up to 21 days.
It also covers all holidays in the UK, provided you are spending at least 2 nights in accommodation you booked before you started your holiday.