For a trip of this length it's an easy answer. Buy the annual policy. It will be cheaper for this hire and will cover you if you rent again later in the year. Most annual car hire excess insurance policies will cover any number of car hires of up to 31 days each and, in some instances, up to 65 days.
You are likely to find an annual policy is cheaper than single trip policies for trips of around 17 days or longer. When you add up the cost 17 days worth of a daily single trip policy it will come to more than the cost of the annual. If you are going on multiple trips across the year and will be hiring a car for these then you may find it better value in the long-term to purchase one annual policy instead of numerous single trip policies.
If you have any further queries please contact our customer advice centre either by online chat or over the phone within our office hours and we will be happy to help you.