If you have to make a long journey and don’t have much time, a domestic flight can sometimes be the answer. Sometimes they can even be cheaper than trains, but you have to do your research. As well as finding a fare and time that suits you, check how far out the airport is from the actual town or city and how long you have to check in before departure; if there is a long check-in requirement, or the airport is a long way from the city, flying might not be much quicker than train travel.
The main operators are British Airways, Flybe/LoganAir, Easyjet and Ryanair. You can check out these sites individually, or use a comparison website such as Skyscanner or momondo or kayak. Make sure you understand exactly what each deal involves and that you are comparing like with like.
Special fares are sometimes available if you’re under 26 or a student.