It does depend where you live, as to when you can plant out, I border on caution, mine usually go out about beginning of April, there's still chance of a frost but the garden's sheltered so a late frost doesn't seem to touch them, brother puts his out in March, he might get his first crop a week or two earlier than mine but can spend a couple of weeks bringing then in and out of shed. Last year the first spuds were ready end of July probably later than most but worth the wait. You are right they can go out sooner but you would need to be mindful of a frost.
Mine chit in an unheated room out of direct sunlight so they aren't on a window ledge and don't go leggy. They've been there a few weeks now and only just have shoots and a cold spell will knock them back. Give it a couple more weeks and you'll find the spuds in shops will start to show weak white shoots which isn't a good start. I prefer to have them at home and take my chances.
Potato fairs are usually from January onwards so other people must start chitting early.
Hope you aren't any more bewildered in Southampton
Brother has a weather station too which helps, he knows when it's likely to be frosty