This question was answered on 8th June 2016
Since 2014, the number of UK premises with 4G coverage has increased by 17% - to 90% in 2015. In addition, more than 99% of UK premises have mobile coverage (Ofcom 2015 Connected Nations Report,
http://stakeholders.ofcom.org.uk/market-data-research/market-data/infrastructure/connected-nations-2015/). Coverage will improve further as mobile operators upgrade and extend their network to meet the licence obligations arising from the Government's December 2014 agreement requiring each operator to provide voice coverage to at least 90% of the UK's landmass by 2017.