I believe for public transport the CAA specifies 600ft cloud ceiling and 1500m vis for twins with an engine out climb rate of 150ft/min and below
Not quite. Under EU-OPS, and for UK CAA public transport, the minimum RVR depends on the height from which a net flight path that clears the obstacles can be constructed.
Code:
Height RVR/visibility
< 50 ft 200 m
51-100 ft 300 m
101-150 ft 400 m
151-200 ft 500 m
201-300 ft 1000 m
> 300 ft 1500 m
Thus if you have an aircraft that, once it has its gear and flaps up at 200 ft, can continue on the required flight path on one engine (and that makes assumptions about the SID etc. -- the standard requirement is 3.3% or 205 ft per nm), then the minimum RVR would be 500 m.
Under EASA ops (for non-commercial ops as well), an LVO approval will be required for take off in an RVR less than 400 m, but there will be no other mandatory specified RVRs.