And AFAIR there's a maximum altitude for certain ground stations too -- is it 3000 ft for tower/AFIS/AG? (Where is DOC listed, BTW? I can't find anything in the AIP or CAP413.)
Extract from UK AIP GEN 3.4.3 (where all the relevant bits can be found).
Use of VHF R/T Channels
..............In the
case of TWR, AFIS and A/G facilities, communications on these frequencies should be restricted as far as possible to
heights up to 1000 ft in the immediate vicinity of the aerodrome concerned and in any event within 10 nm and 3000 ft.