The majority of hedgerows date from the enclosures of the 18th and 19th centuries when strip cultivation was discontinued. However, some are older, extending to Saxon times. It's been estimated that once in every 100 years a new woody species is added to a 30-metre length of hedgerow. See Dowdeswell's classic "Ecology, Principles and Practice" (1984, Heinemann) for a large number of practical ideas on this theme.
Samples of soil from conifer and (say) beech woodland could have their humus content, drainage, water retention and pH compared and sixth-formers could investigate the distribution of herbaceous plants according to these variables. It's an opportunity to apply the statistical techniques taught in A2 courses.
If you are within "school journey" distance of Falmouth, then the new maritime museum (email: email@example.com) has a fabulous interactive meteorology gallery which features scientific explanations of folklore such as "When swallows fly highI" Canaries are no longer used in coal mines, thanks to Humphrey Davy's eponymous lamp. Not so well known is Michael Faraday's contribution to the invention; James Hamilton's excellent biography Faraday:The Life (2003, HarperCollins) reveals all, including the great Victorian's membership of an obscure Christian sect (Sandemanianism) This is useful reading for KS45.
The RSPB (tel: 01767 680551) has lots of information about birds. It has regular surveys of native bird populations, frequently involving the media.