Sweat after exercise and swimming also break down SPF on your skin, so you’re best to reapply afterwards. SPF also degrades over time, so it needs to be topped up.
The best way to know when to reapply? Check the UV index on your weather app. The higher the UV, the more often you should reapply. On particularly high UV days (which are pretty rare in the UK), it’s best to avoid peak sun hours in the middle of the day.
We’d suggest 12-4 on the highest UV days.