Uk people, what's the latest this time of year that it's actually worth going sunbathing?

I am planning to go to the beach after work. I should get there for 5 pm. Is it worth going of you just care about getting the sun? There are always too many 20 year old girls in bikinis dancing by that time in the afternoon so I don't want to put up with all their nonsense unless I'm definitely going to get a tan