First off Lara, you are doing really well. As long as what you have sown has plenty of light they won't go leggy. Prob not too warm in the loft either.
Let the beetroot develop its next set of leaves then pinch out the weakest in each loo roll. Let them grow on until you see the roots coming out the bottom. Then they'll be big enough to plant out.
Same for the lettuce if you sowed a couple of seeds per loo roll.
Same again for the peas, as long as roots are not coming right out the bottom,they are getting enough light and are not too warm then they are fine where they are. None of what you have sown will suffer if its still cold when you plant them out. (After hardening off first of course)
I start almost everything off indoors first.
I mostly start small seeded things(lettuce, rocket, spinach)off in small trays and when they have their first 2 true leaves i pick the best to pot into modules, easy to handle seeds (calebrese, cabbage,beetroot) put one seed per module. Big fat seeds (pumpkin, courgette) go straight into little pots.
So here is what i would do with your list:
Calabrese Monterey F1- start now indoors/undercover (not scared of cold)
Pumpkin for Halloween - wait until Mid April (dont like it cold)
Courgette Green bush- wait until Mid April (don't like it cold)
Parsnip Gladiator F1 - sow onto wet kitchen paper in a sealed container
Perpetual Spinach - start now indoors/undercover (not scared of cold)
Leek Kenton F1 - start now indoors/outdoors (not scared of cold)
Lettuce Cuore - start now indoors/outdoors (not scared of cold)
Carrot Early Nantes - 2 and 5 You could give a first sowing a try now outside in situ.
Cabbage red Drumhead - start now indoors/undercover (not scared of cold)
Rocket- start now indoors/undercover (not scared of cold)
Salad Mix- start now indoors/undercover (not scared of cold)
Generally what is not scared of the cold, should not be kept too warm when you start them off.
The pumpkin and Courgette would be frost tender therefore you are trying to time the planting out to happen around end May, so don't start to soon.
The only thing i would say to get a move on are the leeks, I find that you get your best leeks if you can get them in the ground May/June.
Of course its all dependant on how much space you have indoors.
You will get to learn all the ins and outs even though at the beginning it seems endless!
Hope that helps
Enjoy!