Shared Networks not the Panacaea
A series of similar fixed BB Government incentives are distributed by local authorities to ISPs (http://www.theregister.co.uk/2011/05/27/broadband_rollout_devon_norfolk_wiltshire/), so I can't see why a similar approach would not work with mobile BB? An operator with some local infrastructure/RNC/BSC would find it cheaper to tender to fill in the areas not yet covered.
As for those suggesting making these sites shared, are you suggesting that the lack of a business case in the past 20 years for any of the operators to cover a patch of land/road/rail line would suddenly be overcome to the extent that you can have a free choice of operator because they all deploy there???
In any case NNMBYs (Nowhere Near MBYs) would object to the planning applications that the larger structures require for shared sites would entail, and then would complain that a site disguised as a scots pine was inappropriate in a national park, it should be a douglas fir (sub-species Carnt beyarsed).