Your choice, however:
The British Association of Dermatology (BAD) says that an SPF between 15 and 30 will suit most people and will provide a good level of UV protection if applied adequately.
There are products, such as P60, that advertise themselves as offering all-day protection if applied first thing in the morning. But a spokesperson for the BAD says "Most dermatologists agree that sunscreens with an SPF of above 30 don't offer a great deal of additional protection to those of SPF 30, and can be harder to apply and more expensive."