Yes it is too cold yet.The foliage is susceptible to frost.You need to chit them now,if you haven't already done so,in a cool,light,frost free place.Probably towards end of March for planting out will be ok.You will need some heat to germinate toms and salad,not sure about carrots..outside later,I think,and no,I wouldn't advise heating a mini,plastic greenhouse,if this is what you are referring to.I'm sure the more experienced members will give you more info.

i've never planted potatoes in tubs before,the right time to plant is around saint patricks day 17th march first earlies then 6 to 8 weeks later second earlies and the same for maincrop and as bloomer said start chitting them now use cardboard egg boxes. on the subject of potatoes and tomatoes don't plant them next to each other as they are from the same family (surprised) so was i they can carry the same disease blight ,and you don't want that sow tomatoes for greenhouse now til march for outdoors sow march to april when handling don't touch the stem the acid in your fingers will kill it off always handle by the leaves. p.s remember tomatoes have two root systems.