Probably best to contact Exeter and ask. Law is one of their stronger departments and so might be less likely to be flexible compared to (many) other programmes there. Officially they do list BTECs as an considered subject, with requirement DDD, so if you are predicted D*D*D* then in principle I should imagine you would be considered.
I think for some unis (e.g. Oxbridge, LSE) it might be a weaker preparation for the course (and thus be less competitive) because it's less of an academic course than A-levels, and law degrees (both in general but especially at those three unis) is a quite academic degree programme (inspite of the connection to the vocations of solicitor and barrister, it is not itself a vocational course). However those are among the most competitive law courses in the UK (LSE probably is the most competitive law course in the UK) so that won't necessarily translate to other unis.
It's worth bearing in mind the Russell Group is a) a research consortium, and so membership has no bearing on undergraduate teaching or study and b) comprises a fairly wide range of unis, some notably more competitive than others, for various subjects. So one RG uni might accept you, while another wouldn't equally some RG unis might accept you, but some non-RG unis might not. There isn't really any useful information contained in the RG tag so it's not really worth sparing a second thought for honestly.