'Russell Group' is a self-appointed marketing strategy in which members are frequently taken at their own estimation. There are many universities which are excellent centres for research but are not members.
Metrics can only tell you so much. REF scores are all likely to be high at RG institutions but not exclusively so, and some outside this group would absolutely be worth considering. TEF would make an interesting comparison; great researchers are not necessarily great teachers. As far as the NSS is concerned, some institutions will even make time in the lectures students have paid for to ensure these surveys are completed. They will give rewards, cake, money donated to charity etc, for completing them. Before filling out the survey students are also likely to receive a few extra PPTX slides in their lecture presentations, telling them all the wonderful things the university thinks it has done for them. Ie. 'you said, we did ....'
HE follows a cut-throat, competitive business model these days which unfortunately, IME, has little to do with the real purpose of education. As far as high quality general teaching and research staff are concerned, mediocrity is often rewarded. Those who can't cut it in research often progress up the management chain, and then try to hold back those who are strong on research. Bear in mind, too, that 'buying' prestigious 'best university' awards is a likely possibility. Many VCs are well connected to the establishment.
I'd go for a university with a solid reputation and where you think you'll be happy (there are many of these outside RG). The surveys and metrics the sector is drowning in are only an indication, and IMO should be taken with a large pinch of salt. The stats might well not reflect the reality of the situation.