RH might not be a very 'good' university (whatever that means these days) but their philosophy dept is actually strong, especially for 'Continental' stuff and the relationship between anglophone analytic phil and its Continental counterpart - I think Dan Whistler is still a Professor there and he's a leading authority on Schelling, for example. He is very, very good. They also have people like Rebecca Roach, Oli Spinney, and Neil Gascoigne, all very, very good.
As with anything when looking at these programmes, it ultimately depends what you want to study and what you want out of that study. The strengths of Holloway might not suit you, and in that case, looking at the course content elsewhere would be beneficial. See what grabs your fancy, piques your interest, tickles your pickle about the curricula elsewhere: first year is broadly the same across all institutions, and you'll get a grounding in logic (including propositional logic, usually), Greek phil, and early European phil, so second and third year options are really what you want to pay attention to.
With a view to future DPhils and PhDs, your undergrad uni won't make a huge difference so long as you get a first and a distinction at Master's (Oxon and Cantab are more particular on this stuff than the majority of other institutions). I was going to go on one about the job market and how difficult it is to break into full-time work, especially in good departments, but there is plenty of time for you to worry about that and you might be like me, who knew all this and just went ahead and did it anyway!
For some reassurance re grades, I have poor A Levels, including a fail. I eventually read at a good phil dept at a middling RG institution, where I completed three degrees including a PhD with one of the leading authorities in the world in my research area. The grades are not necessarily a be-all end-all, and there will be good opportunities in your first year to really get to grips with what academic writing is like at university (much different than at A Level, and all the better for it). My advice here is twofold - don't sell yourself short; don't get hung up on 'what if I can't do it'. I've also read at Oxford for a postgrad (and might yet read again for another later this year) and it's much like any other university in terms of the mechanics of things and the workload at postgraduate level.
My own 'letters' just so you know I'm not talking total crap: BA (Hons) Philosophy, MA Philosophy, PhD Philosophy, PGCE Religion, PGCert Education