A huge mobile shake-up has just kicked off in Britain. Telefonica has agreed to sell O2 to Hutchison Whampoa, the owner of Three UK, for £10.25 billion. We knew it was likely, after the pair entered "exclusive negotiations" back in January, but now it's essentially a done deal. Provided the purchase is approved by regulators, it means the UK's four major networks will soon become three: Vodafone, EE and the combined might of Three and O2. The confirmation comes by way of Telefonica, although the company is saying little beyond the amount of money involved. For instance, it's unclear if Hutchison Whampoa plans to maintain O2's brand and services in the UK, or merge them entirely with Three.