The league tables are only useful to a point, as they are for schools, only more so. They are various amalgams of student satisfaction, research scores, employment statistics and entry standards etc.
Like schools it rather depends on what your child is looking for. Some of these factors might matter more than others
Entry standards reflect demand, which may not entirely be based on the universities current reputation with employers or in the academic world (and they can differ). It is also influenced by parental prejudice preference and student's cultural needs (the bar, sports, the city, sloaneyness - I can't see any other explanation for the popularity of Exeter over other similar unis!)
It is probably more important to balance the other league table factors and reputation with looking in detail at what courses are on offer, and the environment and where your child can realistically hope to get an offer.
We found visiting useful, like schools you do get a feeling if it is right. Although open days are like mass zoos (where you get a slightly spooky chance to see genetics in action!!), you do get an impression, and a chance to meet the academic staff. If you are still keen you should visit again when its business as usual. At two of the "top" unis for Science we were completely put off because one course was shambolically organised with modules from other courses bolted together piecemeal but the course tutor couldn't convey any sort of coherance or rationale. It was clearly going to be frustrating. At another the Professors and Teaching staff were uniformly uninspiring, and clearly disinterested in the students as people , they were just a means of accessing the maximum number of A*A students (they sifted applications by computer). Ironically the latter ended up offering my daughter a scholarship, whereas just showing some interest and spark might have been far more effective in recruiting her, a subsequent visit just confirmed her first impressions.
It may take just one inspiring academic in an institution, or a chance to benefit from their research specialism, think how inspiring it would be to go to Durham to read Physics at the moment with their part in the work on the Hadron Collider.
Not all top unis are Russell Group, there is also the 1994 group with St Andrews amongst others.