If you are growing tomatoes in the UK at the moment I recommend that you keep them out of the rain by providing them with some shelter during the day.
Of course, plants can't be left out overnight until the end of May - after the lost expected frost - but they also should not be allowed to stand outside in the cold during the day in the rain!
Sorry to be so direct about this, but if your lovely plants get cold and wet ... it will probably do them a lot of damage and they will be well on their way to a fungal disease!
It is now too late to start tomatoes from seed in the UK because the season (summer) will be over before the tomatoes mature, but if you still haven't started yet, tomato plug plants are the answer.
The low light conditions we are having in the UK at the moment are making tomato plants "leggy".
It is best not to water unless absolutely necessary when temperatures are cold and there is poor light, otherwise plants will become unwell (wet, cold soil makes them very unhappy) and leggy.
Hope to be more positive next time and also that the weather improves!