At the moment, EE is the only carrier to offer LTE (or 4G) in the U.K. Even though Orange and T-Mobile are part of EE, you must go on an EE specific contract to avail of the 4G speeds. This also comes at a premium and with it only reaching certain cities around the country, some may say it is not currently worth getting an EE contract for 4G just yet.
Three UK has recently announced that when 4G comes to their network, it will both new and existing customers will be able to get 4G without any extra premium. In their Press Release, they state:
LTE, otherwise known as 4G, will be added to Three's Ultrafast network later this year. Unlike some other UK mobile operators, it will be available across all existing and new price plans without customers needing to pay a premium fee to ‘upgrade'.
All U.K. BlackBerry Z10 devices are 4G ready, so whenever your carrier hits the 4G switch, you don't need to swap it out for a 4G version. And if you're with Three UK it seems it will be smooth switch. Three even says you won't even need to swap out SIM cards.
Three UK are known for being value for money, being the first U.K. carrier to scrap the fair usage policy and gives customers truly unlimited data bundles. So, could we being seing unlimited 4G data bundles? Could be. Couple that with the fact Three UK is offering very competitive prices, I may be jumping over to the network when they flip that 4G switch. There's no date yet other than it will be coming later this year.
Source: Three UK