When should I repot and plant out supermarket miniature roses?

[BusyBee]

I have 5 miniature rose plants that I've bought from the store in mid-December up until the other week. I've been regularly watering and deadheading them. Some of the older ones have lost their blooms and are now putting on some beautiful new foliage. They're looking a little root bound and I know there are a few plants per pot, but I'm just wondering if I should separate them and plant them out yet? I'm in North West UK and it's still a little chilly here at night.

Also, should I just soak the roots in water to remove the compost and separate them out that way?