According to Cellular-News, the merger will account for cost savings of $730 million dollars/year from 2014 forward, but between 2010 and 2014 the carriers will need to spend $1.2 billion dollars in integration costs to realize the savings down the road. But those integration costs are offset by Capital Expenditure Savings of $1 billion between now 2014 and then $100 million a year thereafter.
Long story short, this gives both T-Mobile UK and Orange UK a much better position financially, which they can leverage to provide better products, at better prices, with better penetration, through better advertising/promotion. You know how the cycle goes.
For more details on the merger including naming of executives, read the article at Cellular-News.