From a pilot point of view, if both aircraft are in level flight then there won't be an RA. We would get a TA "Traffic Traffic".
Flying out of London City this morning at 3000' IFR, we had a light a/c at 2,200' VFR (?) pass pretty close to use horizontally. I think the base of the London TMA there is 2,500'.
We had TA but no RA.