the 800mhz 4g LTe is the best frequency for rural coverage and will penetrate buildings better than the 2.6 4g lte and 1800 4g lte.
As stated with the 98% indoor coverage obligation lot o2 will provide the best 4g for rural areas. This along with the cornerstone agreement with vodafone will see more o2 sites.
To do the upgrade to 4g o2 has to either put in new base equipment that will also provide 3g to areas only previously 2g or if already installed do a software upgrade of the base equipment.
If you look at the o2 coverage checker and enable 4g look outside of the london borughs and you will find o2 have not just launched in 3 cities but also many towns around london that are technicaly nothing to do with london eg woking reigate redhill etc o2 have not realy given a full launch picture.
But lets see how it dvelops
GLOBALNET now part of madasafish now part of plusnet With The same isp since 1995 ( now all part of BT) (now on a 21cn exchange wwstal 50db on adsl2 or 53.5db on adsl2+ depending on exchange sync)