Green light with no arrow means all directions can proceed if it is safe to do so.
Green arrow alone means only turning that direction can proceed (obviously again if safe to do so).
Green light + green arrow right is normally indicating to right-turners that traffic from other directions has stopped and so it should be safe for right turners to proceed - wheras with the green light only, oncoming traffic that might block right turns was being allowed.
As ever, the primary reference point for driving in the UK is the highway code, although this use is more "customary" than legal and is not clearly covered, the closest section indicating
A GREEN ARROW may be provided in addition to the full green signal if movement in a certain direction is allowed before or after the full green phase. If the way is clear you may go but only in the direction shown by the arrow. You may do this whatever other lights may be showing.