3 possible causes - the first is simply the variety you're growing - some do have thick, tough skins. Second is watering problems - if the watering is intermittent and the plant doesn't get enough at times, skins thicken on tomatoes as a response. The third is temperature - exposure to high temperatures can cause the fruit skin to thicken up - its the plant's reaction to moisture loss because a thicker skin means the moisture is preserved inside the fruit.

Here in the Arizona desert, all of the tomatoes have thick skins, due to the heat and low humidity. Of course, the latter probably isn't a problem in the UK, but the former may be a problem if the greenhouse is insufficiently ventilated.