Help! Where have I gone wrong with these onion sets?

I’m new to veg gardening this year and ordered some seeds for starting in spring. These onion sets said plant on receipt so I potted up in a window and they started really well but have now gone super leggy and limp. I know that they would need hardening up outside before planting out but it’s still too cold (I’m based NW England) – have I completely ruined them? Any advice for a complete newbie?