From the air pilot GEN 3.4.3 Para 2.2 (a) & (b)...
http://www.ais.org.uk/uk_aip/pdf/gen/10304.pdf
"In the case of TWR, AFIS and A/G facilities, communications on these frequencies should be restricted as far as possible to heights uo to 1000 ft in the immediate vicinity of the aerodrome concerned and in any event within 10 nm and 3000 ft."
The official line is a tad unreallistic and show the limitations of frequency allocation. As an ATCO I always appreciate a brief hello from a frendly voice when things are quiet in bad wx (just don't disturb the crossword/countdown/cards etc)