It might be useful to see Universities as either selecting or recruiting. Those that select will have more applicants who meet published entry criteria than places. For example in 2016 LSE BA History had around eleven applicants to a place, and is a course that has not been in either adjustment or clearing for a number of years. They will be rejecting good applicants who meet their published criteria.
One trick is to search a subject in the Complete University Guide by entrance standards
www.thecompleteuniversityguide.co.uk/league-tables/rankings?o=Entry+Standards&s=History
A successful applicant to Durham history has an average of 570 UCAS points - so some way beyond 3 As at A level.
Four years ago when my son was applying for economics - with a 4A* prediction, a fifth A level and a language at the equivalent of AS - he received three rejections: Cambirdge, Warwick and UCL. It was far tougher than we had anticipated and it does no harm to go into the process aware that places at top Universities can be extremely competitive.
It is then well worth becoming familiar with the course and considering what they might be looking for, especially those courses who don't interview. For example one boy we know gained a place at Oxford to study history, but not at LSE. In retrospect he thinks the LSE course may have been prioritising those with a stated interest in economic/political history...as opposed to, say, archaeology. And making sure the PS reflects the course content of the Universities applied to.
Once outside the top tier, places are easier to come by. There are then two approaches. Either try one "reach" three achievable and one fall back, or similar. Or decide that given there is an element of lottery when places are awarded for the very competitive courses, to apply to three or four very competitive places and hope you get one.
You then need to expect a long wait. Other than Oxbridge, over-subscribed courses often wait till after the January deadline before allocating many of their places. It is really not unusual to be waiting till late March before hearing from somewhere like Durham or LSE.
That said there are advantages in having studied at a top ranked University and though it is tough to get a place, it will probably be worth it. Though as a parent, the surprise has been just how hard modern students expect to work. Even friends who went to Oxbridge say they did not study through their vacations in the way their children do.