Hi Anne. There are different types and levels of protection. All wild bird species (including their nests and eggs) are protected with some exceptions. The following link gives more information http://www.rspb.org.uk/policy/wildbirdslaw/birdsandlaw/index.asp This is going to be relevant to the habitats of typical farmland birds such as corn buntings, linnets and tree sparrows.
Some areas of bird habitat receive special protection under national and/or international wildlife legislation. This is because they have been judged to be of special importance for particular species or groups of species . Many of these sites will include farmland habitats, although it is difficult to say what proportion. See also http://www.rspb.org.uk/policy/sites/index.asp
At the highest level, there are sites protected under the EU Wild Birds Directive - these are known as Special Protection Areas or SPAs. These sites are designated for particular bird species - those listed on Annex I of the Birds Directive e.g. stone curlew, and migratory species e.g. ducks, geese and swans. There are about 250 SPAs in the UK at the moment. There are other sites, known as Special Areas of Conservation (SACs), which are protected under sister legislation known as the EU Habitats Directive. Again these are only designated for particular species and habitats listed under the Habitats Directive. The Directives provide very strict levels of protection afforded to the wildlife of SPAs and SACs.
At a national level, there is a network of sites known as Sites of Special Scientific Interest (SSSIs)(known as Areas of Special Scientific Interest or ASSIs in Northern Ireland). These are notified under relevant national legislation, for example the Wildlife and Countryside Act 1981, or the Nature Conservation (Scotland) Act 2004. Many of these will designated for their bird interests. These can cover a wide range of bird interests, ranging from very rare breeding birds, wintering birds and different breeding bird communities e.g. lowland wet grassland breeding bird assemblages. In the UK there are about 6800 SSSIs/ASSIs designated. Many of these sites are also designated as SPAs (around 75%). A large proportion of the habitats on these sites will be farmed in some way.