Your plants will survive above freezing, but growth won't actually
occur until about 10 C degrees or so (about the low 50's I think so
you should be OK). More for most cultivars of lavender. Pansies will
go to about -7, and most of my spring bulbs are fine with temps down
to -5C or so at night, but I wouldn't try that with your tomatoes.
Consider springing for some lights next year. You start plants early
to give them a head start on growth, and putting them out in marginal
temperatures doesn't let them do that. Yet it's very awkward to cover
them at those temperatures because the plants can heat up quickly and
do themselves in. I lost a whole tray of plants last week because I
didn't vent my cold frame. I left in the morning when it was about 0C,
and the temperature went about 12C in the afternoon. By the time I
came home from work they were just fried! You can get row covers
though that are permeable and continue to allow air circulation.
If your plants are looking a bit peaked, I wonder if your plants were
hardened off before you put them out. They can basically go into shock
if just suddenly put outside for the entire day.