They look like earthworms, the 2 on the left appear to be lining up to mate. they have their 'saddles' next to each other and are head to tail with each other. Nothing to worry about they will have been hiding and overwintering in there. They will be feeding on the compost in the planter. The smaller ones are probably juveniles. You can tell if they don't have a saddle they are juveniles. Don't know which species as there are several in Britain.

I tend to agree with Sbg,about Earthworms..too big to be brandling ones,as they usually live in the compost bin .Anyway,they will be doing a good job for you..

yes and worms actually mate with themselves (hermaphrodites).

They are indeed hermaphroditic but they don't mate with them selves they are self sterile. They line up head to tail [the ones on the left hand corner are doing just that]so their respective male and female parts are opposite each other and then they both transfer their sperm cells into each others female parts.
very efficient!