Which insurance policy you buy really depends on which best suits your needs - there are three to choose from;

Third Party Only (TPO)

Third Party Only (TPO) is the legal minimum you need to ride a bike. It covers any damage to other people, their bikes and property and will pay out to repair this damage. However, injury to you or damage to your bike or property is not covered. The cheapest of the insurance options, but you get what you pay for.

Third Party, Fire & Theft (TPFT)

Third Party, Fire & Theft (TPFT) is the next level up from TPO and covers the damage to other people, etc as above, but also covers your bike in case it's stolen or burnt out. Slightly more expensive, depending on where you keep your bike and what security it has, but still not as expensive as fully comprehensive.

Fully comprehensive

Fully comprehensive is the top level insurance. Other people, bikes and property are covered, your person and bike is covered (obviously depending on the exclusions within the policy - i.e. you may not be covered to ride track days, always check before doing anything other than usual riding). The only downside of this insurance is that it's the most expensive.