DS1 split his uni visits between me taking him on weekends, his father in the week and Oxford and Cambridge visits organised by the school. We did them all summer Y12 autumn Y13. He went to a couple on his own as well. Pretty intensive.
He looked at:
Oxford
Cambridge
Warwick
Nottingham
Southampton
Cardiff
Exeter
Birmingham
He ruled out Bristol and Bath as we have family there and he felt they would be too close by. (Silly reasoning, but oh well.)
We ruled out London as too expensive.
He also attended offer holder days at Exeter and Southampton to decide which to choose as insurance.
With DS3 we'll look much more closely at accommodation costs and whether you get uni accommodation at your insurance.
DS1 was much more concerned about the variety of options for the course and the general feel of the campus/city uni buildings. He didn't care much about accommodation position/cost/facilities. He couldn't imagine what living away from home would be like. If he was doing it again accommodation would be a higher priority. He liked modern campus buildings over traditional ones! 
I'm not going to start looking with DS3 until I have a better idea of his predicted grades. No point rocking up at Warwick or Bath if he's predicted AAA, no point looking at many of the above if he's predicted ABB. We'll know better in the summer.