Real Scrumpy in London?
Is there any to be found, and if so, where? What are some
Mass produced brands to avoid?
It tends to go hand in hand with real ale. The CAMRA guide has a special section/ rating for cider and perry houses so I would suggest you hie thee hence. Also any real ale festival tends to have representation - including the option on 1/3 pints for the more formidable ones. Although there aren't many large beer festivals in London, there are often ones at large pubs though you may end up having to make a short train journey.
I'm not sure if anywhere in London does scrumpy as I remember it growing up which was basically rocket fuel in large plastic containers. Not sure I'd want to try it now either.
If however you are looking for a range of good ciders I'd head to the Cider Tap which is opposite the excellent Euston Tap.
I'd second the cider tap, and also recommend The Southampton Arms (http://thesouthamptonarms.co.uk/) in North London.
I like cider but I'm not a buff, so I can't recommend any scrumpys I'm afraid.
As for mass-produced scrumpy, the only one I can think of would be Scrumpy Jack, but that's not really scrumpy. Plenty of mass-produced ciders such as Bullmers, Magners, Strongbow...