That depends on who owns the freehold and the constitution of the management company. If the flatholders jointly own the management company and the management company owns the freehold, you can convene a General Meeting and change the terms of the ground lease. The Articles of the management company will tell you how many have to agree. If it has to be unanimous, you may have problems convincing everyone to agree.
If the freeholder is someone else, you're going to have to convince him to let you join the 21st century.
A large communal dish and distribution system will give better results than you each having a small one. You'll have to work out who meets the up-front costs, where you keep the electronics and who maintains it. Again, if you have someone in the block who's happy with the terrestrial channels, you'll have a battle on your hands.