The deadline to ensure that your application is given equal consideration by your uni choice/s is 15 January for most courses. The exceptions are Oxford and Cambridge (all courses) and Medicine, Dentistry, and Veterinary Medicine/Science (all universities), for which the deadline is 15 October.
What 'equal consideration' means is that unis must assess all applications received by the stated deadline in a fair way. Applying significantly before the January deadline does not give you an advantage (except, possibly, that if you are very lucky and all your decisions come through early you will have a distraction free January exams period). While some unis make offers very quickly to early applicants (but will also reject some), others, for some courses, specifically hold over all applications until after the deadline. It's been known for people applying in September to wait until March or even April for a decision and then get a rejection. The other point to bear in mind about applying really early is that you increase the risk of feeling uncertain later about your course/uni choices, where waiting a month or two would have given you more opportunity to make sure that your choices were the right ones for you.
If you are going to apply after the equal consideration deadline, the basic procedure is the same, so you may wish to read the notes on UCAS Apply. However, there are some additional points you need to consider, so as not to waste your choices.
Who Can Apply Late
Everyone – whether a Home/EU or an International candidate – can submit an application for immediate consideration up until 30 June. After that, all applications are placed into Clearing - which starts in early July. However, universities can only offer Unconditional places in Clearing, so most applicants will have to wait until after the exam results in August to be considered. In practice the 15 January deadline for equal consideration applies to many international students too, though it may take longer for a uni to fill up the places they've allocated for international applicants than it does the ones allocated for the Home/EU people. [This is why UCAS now has two search engines; you need to make sure you have picked the right one.] The student from the US applying to LSE, or to Durham for History, on 16 January is wasting his/her time, but once you are beyond the top ten and/or highly competitive subjects like Economics/History/Law/English the prospects for consideration even at relatively highly ranked unis can be pretty good for Home/EU as well as International applicants. However, if in doubt, check.
There’s no immediate rush after the January deadline to submit a late application. Late is late, whether the application goes in on 16 January or 29 June. As Unis will be processing the applications they did receive before the deadline, they may not know for a few weeks afterwards whether they can consider any more. So, though it may seem odd, it can be better, at the mid January stage, to wait for a while. By the end of February unis will be notifying UCAS whether they will be in UCAS Extra and if so for which courses. After the end of February it makes sense to get your application in as soon as possible to give yourself the best chance of success, but every year thousands of people get university places having applied so late that they could only go through Clearing.
Checking What's on Offer
The UCAS system now splits UCAS Course Search into two (Home/EU and International), so it is important to select the right database when searching for options. Courses that are in Extra are marked with an 'x'. Those "currently closed" to new applications are marked with a 'c'. Until Extra opens on 25 February, it is not possible to tell from the UCAS site which unis/courses will be in Extra. Anyone can use UCAS Search: you don't have to register on the site, be logged in, or eligible for Extra.
Whenever you decide to apply, it is strongly recommended that you contact each uni department you are interested in and check that they are still accepting applications. Last year, some course entries on UCAS appeared to indicate that they would not accept applications through Extra, but that they would accept completely new applications. Seems an odd distinction to me, and really emphasises the importance of contacting unis before submitting an application after 15 January, whichever category you are in. Remember too that the UCAS status may be out-of-date (or even wrong, especially in the very early days of Extra).
When you contact unis, it's probably best to phone them and ask for a specific address to email so that you know you are asking the right person. You'll need to explain your circumstances, your grades or predicted grades, and tell them a bit about yourself. They will tell you whether they would consider you, and if they say yes then you can apply. If they say no, then don't waste your application, and find somewhere else.
Extra opens on 25 February 2013: use the regular Course Search pages but remember to check the 'extra courses only' box at the bottom. The Course Search for 2012 (Home/EU) is here but remember that this reflects the position when Extra closed at the end of June 2012. Many other courses will have been available in March but full by June.
All other aspects of the application process remain the same. You will need a good PS – the PS Help forum is still open after 15 January though you may wait a little longer for a review than previously – and a reference. The general advice in How to Avoid Getting 5 Rejections applies to late applicants too.
If you apply between 16 January and 30 June you can make up to five choices; if you are unsuccessful and/or decline any offers you do get, you will become eligible for Extra and/or Clearing in the same way as other applicants do, depending on the timescales.