As the names suggest,
single trip travel insurance covers you for one trip abroad, while
annual - or
multi-trip - cover will insure you no matter how many times you go away in a year. Given that you would pay around £25 per trip for insurance, if you go abroad more than three times in a year, you are likely to save money with an annual policy. The one thing to be careful of is that you do not exceed the maximum stay abroad allowed by your policy - it can be 21, 35, or 45 individual days away that are covered in one go.
Tempting though it is to have the day you start your holiday as the day your policy starts, if you need to cancel the holiday for a reason, such as a death in the close family, you would not be able to claim. So it may be worth starting the policy from when you book the trip away.