Outside the former GWR, Relief tends to be a 3rd Line designation after Fast and Slow, where the lines are all passenger rated. As swt_passenger says, there's relief lines at Waterloo throat, the southern WCML also has them at Willesden (Up & down reliefs connecting the West London Line to the Slows), Tring (platform 4) & Bletchley (Relief 1 & 2 are between the slows and the sidings).
A very odd situation is at Oxford, where the loops were extended to form a 4-track to Wolvercote Junction and so it became Main and Relief, with a convention arrangment of DR-DM-UM-UR (although in the appendix they are only labelled as Up/Down Oxford, not 'main', but I've seen other documentation that refer to them as mains), but because of the platforms only being on the Relief lines they raised the RL linespeed and put in high-speed points where needed. Freights are intended to be held on the Main Lines, and passengers to overtake them via the Reliefs. It makes sense when you think about it, but it looks odd on a plan having trains waiting on the 'main' for the 'relief' to overtake