I'd guess you'll be fine, so long as there are no public transport strikes in the UK (that's right after the Olympics, so I would guess that the Government will be keeping the unions in line then) and there's not some bizarre AF strike impacting intra-European routes but not TATL ones. Should be one of the easier CDG transits to make, since I don't think they use buses on flights inbound from LHR since the UK is not in the Schengen zone.