GetAHairCurl - fine if the course is not oversubscribed. However for the top 3-5 courses in popular subjects the basic entry requirements will normally not get you in unless you offer something else (eg doing well in a poor performing school). Having a look at the average entry tarriff can help as it gives a sence of the standards reached by the rest of the field. For example the average UCAS points for UCL, Warwick, St Andrews and LSE for economics hovers around 550. Relatively few, I expect, will have lots of music and drama awards, so most sucessful entrants are offering more than 3A*s at A level.
I am not convinced that this is discriminatory in the way that some seem to. DC were at a high performing school. (Better results than Eton!) DS was rejected by Cambridge, UCL and Warwick despite a 4A prediction and indeed achieving 700 worth of UCAS points (though is now doing very well at LSE). I am sure that other candidates will have been admitted who were only predicted the standard AAA entry requirements, and this seems reasonable. Universities are lookng for potential. If they are oversubscribed, they won't necessarily want to raise their minimum entry requirement, but will keep the pool wide and then sort by other factors including contextual data, aptitude tests including STEP, interview and personal statement. It will not go all the way to redressing inequality in education, but Universities also want to be satisfied that students have sufficient knowledge before arriving at University to cope with demanding courses.
FWIW Imperial seem to vary their offers on a candidate by candidate basis more than most. They usually interview and seem to give out individual offers, even for pupils from the same school and for the same course. These offers, and indeed some Cambridge STEM offers, will often be over four A levels and include more than one A*, though presumably a good candidate taking three A levels would get an offer based on three A levels.
It is accepted that if you want to get a place to study medicine you need to dig a bit deeper and identify how each medical school is selecting, and look to see where you have the best fit. The same probably now applies to top engineering, economics and law courses. Applicants should probably make sure they are not wasting a choice (Oxford may say their standard offer is 3As, but 3As on their own would probably not be enough) and that they do their best with the other factors which will be taken on board, like personal statements (follow any guidance given by the University), aptitude tests etc.