Clair
I am in the same region as you and I found last year that many of my leaf plants, sown samish time as Charles sowed, were more leggy than his. Charles is lucky to have a very sunny aspect at Homeacres, much more than me here, so I think high indoor temp + lower light levels may cause the legginess. So I moved my spinach and cabbage after transplanting into 7.5cm pots out into my lean to in early March and they did Ok.
This year I have put my leaf sowing back to 18/02 (nothing through yet) and the Boltardy to 24/02. I intend moving them into the lean to as soon as the seedlings emerge properly and can grow rather than germinate.
All my earlier beetroot seedlings, even those sown in late April, seemed more leggy than Charles’! But they all gave good crops in the end…..so unless they start falling over I would persevere. Last year I transplanted into the soil in mid March and the plants did much better with a shot of seaweed after sowing to get them to establish well. This year, it may be too cold to do that….