Berwick Street is still good for reggae.
Music and Video Exchange and Reckless Records still have shops there. Plus there's one next door to MVE which has some old sevens. Reckless is probably best - they have two shops and the one you want is the southern most. (the other one is mainly dance music and has a few bashment things in).
Sounds of the Universe is nearby and has a 2nd hand bit downstairs which seems pretty expensive iirc.
Supertone and Blacker Dread are still open in Brixton but not sure if they do 2nd hand or revive stuff.
Dub Vendor in Clapham Junction do new release and revive. (see their website) Ditto their shop in Ladbroke Grove.
North side you are looking at Regal Records in Clapton, Body Music in Tottenham and then some strange stores I haven't made it to yet like:
Bags has moved from Camden and reopened at Unit 56-7, Seven Sisters Market
231 High Road, Tottenham London N15
Hours 11am to 6pm from Wednesday to Saturday
The market is right outside Seven Sisters Tube
Plus some stuff in Walthamstow...