The grid below, only for applicants to undergraduate courses at mainstream universities, indicates important points in time for you to consider if you apply through the centralised UCAS system. Some of these are only guidelines. Don’t worry if you are not up to date with these. You can still apply but you will have to step up the work you do to be successful. Some are absolute deadlines however! These are indicated in red.
The timeline for applicants to Postgraduate courses would be very different, if any would exist at all. Obviously, the timeline for applicants to studentships would depend on when vacancies appear and are more like looking for work than looking for a course. It would be very important to keep an eye out for interesting placements. Preparation for these is not unlike preparing for a job application, while at the same time keeping up your networking and work on your subject to ensure being a strong candidate for entry.
Planning for applicants for Postgraduate taught courses and research courses is more flexible as well, apart from when you apply through UkPass. If you apply to universities direct I would advise you to be early rather than late. From experience I know that for those courses most in demand February can be late already. Other, often equally good courses may still be accessible much later. In my experience a lot of interviews take place in or around April time or staggered throughout spring, but that doesn't mean you can't try to get in later.