You will see many lovely photographs of Dartford warblers (Sylvia undata), they seem to be a favoured species amongst photographers. In spring the males perch on the top of gorse bushes to sing and proclaim their territory and so they can be sitting target for the cameraman with the big telephoto lens! I have no telephoto lens and, actually, are fairly unlucky with Dartford sightings overall so my effort with the camera is a bit disappointing. However, it does represent the sort of view you will get when out walking as they are nervous birds and easily spooked if you attempt to get too close.
To quote the much used phrase of the Springwatch team, the Dartford warbler is the 'iconic' species of the Dorset heath. Dorset is its stronghold along with the New Forest. They do occur on heath elsewhere in Surrey, Norfolk, Staffordshire and possibly elsewhere but if you want to be sure of seeing a 'Dartie' come to Purbeck in Dorset, find some gorse bushes, then wait and hope! They feed on small spiders that thrive on gorse and they nest in the middle of gorse bushed for protection so you will not find a Dartford unless there is gorse close by.