Is this internal or external signal reception
No, if the article is correct, it's simply the ratio of a sample of users connection time by technology. The problem of indoor versus outdoor is actually becoming a bigger problem with new build properties due to the fact that they are now 90% made from foil covered insulation board.
It would be nice if Ofcom would get off their saggy grey arses and specify and approve a suitable picocell or femtocell that phone users could buy off the shelf and plug in to extend a wireless signal indoors, without needing this to connect via a broadband connection, and without needing network operator permission to do so. Then again, it's Ofcom; nothing will happen, and even that nothing will only happen slowly.