Grading is subjective - it can only ever be a guide: and I think you'll find that even the people assigning the grading would say that's the case - it's only to give an approximate idea of the difficulties, technicalities and stamina required to get to the furthest reaches for an average caver.
Grade 1: easy, no pitches or difficulties.
Grade 2: moderate, maybe small pitches.
Grade 3: Caves/potholes without any particularly hazardous, difficult or strenuous sections.
Grade 4: Caves/potholes which present some hazard, difficulty or large underground pitch.
Grade 5: Caves/potholes which include very strenuous sections or large and wet underground pitches.
Bru, there are no grade 5 trips on Mendip (except IIRC, David Heap wrote - before Howard's Dig created an easier approach - that he considered the Black Hole Series of Swildon's Hole to be worthy of a grade 5).
I guess Dallimore's Cave might be rated 5, the Oxford Extensions for example; and maybe the new Grand Circle/Grand Tour of Swildon's Hole should be 5 for the stamina/free diving. Discuss.