The minimums for landing sites are clearly laid down in the UK Air Navigation Order and in JAR Ops 3.
General permissions or exemptions from Rule 5 (Low flying, ability to alight clear of congested areas, flying over gatherings etc) are issued individually to each operation, but amount to the same thing.
Having permissions to go below minima does not preclude you as a pilot from considering third parties and private property when chosing a landing site. In fact one of our biggest worries is injuring a member of the public when landing or taking off.
Of course all of these permissions are only valid when undertaking a primary HEMS mission. (Danger of death to an individual)
There is essentially no difference between the terminology of air ambulance and HEMS, but an air ambulance could also be construed as an aircraft that performs a medical repatriation from another country.
There is certainly no lack of excitement in being a HEMS pilot!