Could this be the Sovereign version or possibly a Daimler? Notice the chrome trim on the tail lights. This is used as the Prime Minister's car so it should be a luxurious version. Unfortunately the registration doesn't seem to check out on the DVLA website.
In recent years the Prime Ministerial limousine has been mainly been a long wheelbase, armour-plated Jaguar. The current flagship of the Ministerial fleet, used to transport Tony Blair, is an X350 Jaguar XJ8 LWB, which is heavily armoured and cost about £200,000, although unlike George W. Bush's chunky Cadillac DTS limo with it's 10 inch-thick doors, Blair's car looks like an ordinary Jaguar from the outside. Apparentely a new Rover 75 LWB version is also used as the Prime Minister's transport, but I haven't seen him in it yet. The ministerial fleet has had Rover 800s and Vauxhall Omegas, and Jags for the senior cabinet members, although there are quite a few foreign cars in there, like the Nissan Primera. It is important for leaders of car-producing nations to patrioticly use their own brands, that's why Putin continues to retain a couple of old Zils in his fleet despite them being rather obsolete by now. Of course the greatest British cars are Rolls-Royce and Bentley, but that would be seen to be too extravagant, especially for a Labour governement, look how much flak John "Two-Jags" Prescott took for owning two Jaguars. Tony Blair uses a Chrysler Grand Voyager (probably armoured) as his private family car for his wife and four chidren. Only Her Majesty The Queen gets away with riding in massive Rolls-Royces, and has a new custom built Bentley as the official Royal car. In the House of Cards trilogy, machiavellian Tory Prime Minister Francis Urquhart (the brilliant Ian Richardson) has no qualms about using the stately Daimler DS420, although it's been many years since British ministers have used that car. Though to be honest the cost of a few modified limos is trivial to the amount of money the government has its disposal, think of how it costs to keep a fleet of warships afloat, but of course politics is (or should be) all about principle.