When will the weather become warm enough in the uk to keep plants such as tomatoes outside without wilting?

Depends on your location, best barometer is to watch Gardeners World on Friday night BBC 8.30 pm. That is based near London, for every 100 miles north of that add a good week to ten days, and even then you might have to grow them in a mini green house and grow bag recommended. When they put plants out then they are usually right!