Which tomato variaty should I get?

For "mainstream varieties" i.e. ones the big seed suppliers sell into garden centres I would recommend:

Sungold (or if you can find Honeycomb this is related and has a great flavour but is less prone to splitting)
Tigrella
Gardener's delight

Sungold/Honeycomb are cherry type and Tigrella is on the smaller salad size with nice markings on the skin.

You're not that late! I sowed mine on the 12th March and ones for outdoors two days ago.