Weather worries :-(

[Newton]

They will outgrow that pot very quickly.....pot them on ASAP before their roots get to intertwined.

By June they will be triffids! You can grow them in large pots but they need lots of feeding and watering.....the ground will be best but the are very hungry for nutrients so the soil needs to be rich with well rotted manure......or liquid feed!

[Likac66]

It's too cold at night still to plant them outside. I have grown courgettes in tubs before but have used very big tubs - I used old recycling boxes, one plant per large box.

You need to wait until your last frost date. Given you are further north than I am, I would guess it will be at least the end of May. I don't plant any of my frost sensitive plants out until the start of June.