what is the explaination for the increased longevity in the british
population over the previous thirty years?

Here's a newspaper article on the subject (may require registering and logging in)

If You've Got a Pulse, You're Sick
http://www.nytimes.com/2006/05/21/weekinreview/21kolata.html?_r=1&oref=slogin

The reasons are pretty much the same for all areas of the world where
life expectancy has increased:  better nutrition  - also at a young
age -, better and safer living and working environments, improved
health care, which includes some major medical advances in the
treatment of cancers, strokes and heart attacks, which were often
fatal thirty years ago.