If you have a glasshouse or poly tunnel of sorts you can plant your tomato plants mid April but be careful of damaging frosts.
Instead of wasting valuable fuel close your growing area in the late afternoon and spray some water on the floor.The evening sun will mist up the area and protect from frosts till the next day.
We do this all the time and it is very effective.
For severe frost cover with black polythene or agricultural fleeces well.

If you do not have a glasshouse or ploy tunnel wait till the weather becomes frost free.

Andrew Ross