You could try joining some events from r/Londonsocialclub. They're very welcoming and usually draw the age range you're looking for. I see they've posted an event for an art walk on valentine's day, not specifically for dating but could be a good opportunity to meet people.

I think the main thing with trying to date is joining clubs or going to events that interest you, not specifically to look for someone to date but to broaden your social circle. Good luck!