Why is it that Health Promotion and Health Education tends to be most
beneficial when delivered more at a local and community level rather
than at National levels? What are the problems faced with National
Health Campaigns?

I suspect the premise of the question may be wrong. Many highly
effective health education prograames are delivered either at the
national level or at a combination of the national and local level.
The current drive to encourage people to quit smoking, for example, is
delivered largely by the NHS at the national level, with high profile,
centrally funded tv and billboard advertisements. True, many if not
all NHS trusts, and Primary Care Trusts employ Smoking Cessation
officers etc, and provide services to help people quit. But these are
surely elements of a national prgramme delivered at the local level.

Community health promotion activities may be successful in a more
meaningful way, however. Healthy eating initiatves are delivered by
local Sure Start franchises and local authorities, amongst others.
Sure Start is particularly appropriate because it's activities are
co-ordinated and delivered locally: many of the staff working for Sure
Start are beneficiaries of Sure Start services, or they know many
children and families that recieve the services. And because Sure
Start is so closely linked to the local authority, they can act as a
single point of contact for a range of agencies that seek to deliver
those programmes. And while monolithic, nationally organised campaigns
may often be seen to patronise the target audience, or use imagery,
language, and style that alienates them, locally delivered programmes
are better at sneaking instruction and education in under the guise of
economically benficial activities. So a daycare centre that teaches
kids that it's good to eat 5 portions of fruit and veg a day may be
more successful purely because people think the dacare rather than the
education is the main point.