I shall shed light on the mech eng situation. This may not be true, but this is what I was told by a friend who's Dad graduated from mech eng at Imperial in the mid 80s.
They have an interview date in October (selected few), November (larger group), December (larger group), January, February and March (Latter three are much larger groups than in Oct, Nov and Dec). In the first three interview sessions, they aim to interview everyone who has got an A* in maths with a good personal statement, people who already have met the entry requirements and people who Imperial would love to give an offer to (many A*/As at GCSE and straight As at AS). They then wait until mid January for Oxbridge to release their results and those who get in, Imperial accept that they'd firm them and decline the Imperial offer and those who don't get into oxbridge but received an offer off Imperial may firm them before mid January. After this, they see how many spaces remain and rank your application from strongest to weakest.
Next step is to issue the strongest of applicants in the list with an interview in late January, and make offers to most of them. Next they issue interviews to the next strongest batch of students in February. Finally they issue interviews to the final batch of students in March (number changes depending on how many offers were given out previously). The remaining applicants will not receive an offer for mech eng. These applications are divided into two groups - those that they think would benefit from a degree from Imperial, and those who'll benefit from an engineering degree. The latter lot are rejected as it'll be in the best interests of the student. The first lot of applications are sent off to other departments who consider these and give offers out for that course respectively. The rest who remain get rejections in the final week of March or the first week of April (Day after Jan exams results day for me).