BBC does have RGB output.
BBC RGB output is TTL ~0-5v
CPC RGB output is not TTL ~0~1v
So you should just be able to add the usual resistors required for a SCART cable.
(I have seen Amstrad Colour monitors plugged straight into a BBC, it does work, it gives a very bright picture, and is not recommended)
The middle pin on the BBC RGB connector is 5v, which is ignored by the CPC colour monitor. Don't try this with an Amstrad green screen, it uses the middle pin for luminance.