Perhaps they didn't get enough water about a week or ten days before the browning? Unless you've got them outside? If so, bring them in for another month. Tomatoes go out as strong plants, not as seedlings. They thrive in Spain, but have to be molly coddled in UK. If you're a beginner and growing tomatoes you're well. Leave them somewhere warm but not to frazzle being hot glass, and give them more time. Show us your tomatoes in September