Minimum Height Rule was Rule 33 of the UK Rules of the Air and was one of the Instrument Flight Rules however; that disappeared on the introduction of SERA where the Minimum Height rules for VFR operation can be found in SERA.5005(f)
(f) Except when necessary for take-off or landing, or except by permission from the competent authority, a VFR flight shall not be flown:
(1) over the congested areas of cities, towns or settlements or over an open-air assembly of persons at a height less than 300 m (1 000 ft) above the highest obstacle within a radius of 600 m from the aircraft;
(2) elsewhere than as specified in (1), at a height less than 150 m (500 ft) above the ground or water, or 150 m (500 ft) above the highest obstacle within a radius of 150 m (500 ft) from the aircraft.
In practical terms, taking the MEF from the chart and adding 1000 ft gives a reasonable VFR operating altitude. The MEF includes provision for uncharted masts and obstacles in close proximity outside the marked area.
surely the MSA should be an altitude you come down to if forced by bad weather,
I think you are confusing this with the SA which is the minimum altitude you descend to when IFR!
For IFR Operation under SERA replacing Rule 33:
SERA.5015 Instrument flight rules (IFR) — Rules applicable to all IFR flights
(b) Minimum levels
Except when necessary for take-off or landing, or except when specifically authorised by the competent authority, an IFR flight shall be flown at a level which is not below the minimum flight altitude established by the State whose territory is overflown, or, where no such minimum flight altitude has been established:
(1) over high terrain or in mountainous areas, at a level which is at least 600 m (2 000 ft) above the highest obstacle located within 8 km of the estimated position of the aircraft;
(2) elsewhere than as specified in (1), at a level which is at least 300 m (1 000 ft) above the highest obstacle located within 8 km of the estimated position of the aircraft.