I think it is easier if a prospective student has older siblings. I assume Goodbye Stranger's DD will have visited siblings and have a reasonably up to date view of Oxford and Durham Universities and presumably what she wants from from her course. It also sounds as if she is very likely to get one or the other, so a quite different position to many.
It was different for us. DH went to Oxford and I went to London, but our information was out of date, we knew very little about life on a campus University and DC's priorities would be different. Even then it was easier for us as we had both been to University and their school was very well informed.
I would really recommend OPs DD going to an early open day or two at nearby Universities, to get a feel of what she does and does not want. This will help narrow down options.
We did a mix. With DS we did self guided tours (there is always someone in the department office willing to answer course questions) of Exeter/Bristol/Bath and then York when we happened to be in the vicinity. I kept an eye out for things that DD would be interested in (sport, sport and more sport) which later helped inform her short list. DS went to London options on his own, as did DD, because they were more of a known quantity. I went with DS to Warwick, mainly for logistical reasons, and then split up so we got to see more. (He did the subject talk, I went to the talk on studying abroad etc.) It was both interesting, especially some of the briefing on how they select and what they want to see in a PS, and useful to see what appealed to DS and what did not. You never know what very quick decisions they may need to make after results come out and the more informed a sounding board you can be, the better. We both went to Cambridge, mainly because I wanted a day out. Again interesting, and useful to know what appealed to DS and what did not. If he had been totally in love with the University I might have suggested he take a year out and reapply.
DD was applying for medicine. By the time we had completed the spreadsheet of places she might want to go to and who might interview her there were only three options, so no need to visit, especially given that they interview. Frankly she was going to have to be happy if anyone offered her a place. Luckily two did. One she liked, and the other very foolishly cancelled the promised post interview tour on the basis that one uber confident interviewee said it was not needed and DD felt unable to object. Choosing which University is a huge decision and DD found it easy to opt for the known, rather than unknown option.
OP don't worry. Follow your instincts, but also remember there may be interviews, and probably offer days. You have five choices and can only go to one. Desk research on the course etc is often more important.