OK, now we're over in GYO
You're doing the right thing with the spuds by leaving them in a cool, light place. They can go in any time from end of March to probably late April / early May (depending on which variety / type they are).
Parsnips often don't germinate well when sown too early. You'll find that packets say they can be sown from Feb. I don't bother before April, particularly if the early spring has been cold and wet, as the seeds can just rot off instead of growing.
The small plastic greenhouse won't provide sufficient thermal insulation for tomatoes / cucumbers if you're aiming to get an early start. In reality, unless your greenhouse is properly heated, it's not worth starting things off early in it because of the risk of frost and stunting any plants that do start off. I see you're in Wigan; it is certainly too early to start anything off yet. I suggest you post a separate question about this in the Growing in the greenhouse board. Sweet peppers are also likely to be far better in the greenhouse than outdoors.