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George Osborne has defended his choice of a £10 burger, which featured in a tweeted picture of him finishing off his Spending Review speech. The "posh burger" is now everywhere.

Next week two US "better burger" chains - Shake Shack and Five Guys - will open their first branches in the UK.

They hope to fill the space between the quotidian fast food of McDonalds and Burger King and the upper reaches of the market.

In the two decades since the term "gastropub" was coined, the "poshification" of burgers has been steadily spreading. Across the UK, it's easy to see burgers on menus that claim to be something out of the ordinary.

The beef might be Aberdeen Angus or "coarse-ground". It's not a mere cheeseburger, but instead "topped with Taw Valley cheddar". Other adornments might include guacamole, harissa or pancetta. You can have your burgers in brioche or ciabatta. The chips are "hand-cut" or "triple-cooked".

In specialist burger restaurants, the meat can be mixed with bone marrow and presented in a juicy, sloppy mass that seems almost unrelated to standard fast-food fare.

And the market for this more rarefied burger has been growing rapidly.

Byron, which supplied the chancellor's burger, has branches in London, Liverpool, Manchester, Cambridge, Kent and Oxford.

Gourmet Burger Kitchen, started by three New Zealanders in south London in 2001, now has 59 UK restaurants.

In London, specialists like Hache and Honest have gained attention and MeatLiquor and Patty & Bun can have queues of one or two hours.

In Manchester, Almost Famous burgers in a venue hidden behind an unmarked, brown door.

Enthusiasts are gripped by the search for the perfect burger. The likes of and other chronicle the search. In Scotland, a whole nation.

"[Gourmet Burger Kitchen] was the first premium burger chain to really make an impact," says William Drew, editor of Restaurant magazine. "It was followed a few years later by Byron [who] added to the market in terms of quality.

"In the last few years a plethora of chains have opened, many in London where there is lots of demand for the premium outlets. People want good value food rather than just cheap and they're predisposed to eat out even more than ever despite the recession."

Even in the hard financial times, the UK burger bar market was worth £2.79bn last year with more than a quarter of adults claiming to visit a burger or a chicken bar at least once a fortnight or more, according to Mintel.

People aged 16-24 were the biggest group of diners but families with children aged nine or younger were the fastest growing group of customers. Deputy Prime Minister Nick Clegg, who has three young children, said he had also visited a branch of Byron, after a trip to the cinema. He has personally endorsed their milkshakes.

But the essence of the fuss over Osborne's burger and chips is that it cost £9.70. There may be some for whom that feels, well, a little steep. Gastropub burgers can cost even more.

Five Guys and Shake Shack will be aiming for a more modest type of consumer.

"Their product is above the fast food giants and significantly cheaper than the premium burgers," says Drew. "While Gourmet Burger Kitchen and Byron charge around £7-£10 a burger, Five Guys and Shake Shack will be selling them for around £5 without fries."

Simon Rimmer, chef and presenter of Channel 4's Sunday Brunch, has been serving burgers on the menu in his restaurant Earle since it opened in 2006. "They're still relatively cheap and combine reasonable prices and quite exciting, quite sexy food.

"You can go out with six people and all have a different experience because you can serve it in so many ways. On Mondays to Thursdays burgers make up about 30% of each bill because it's a cheap way to get a good red meat fix."

For some it's the quality of the meat, "dry aged" or "Wagyu" or some other golden signifier of quality. For others, it's all about the add-ons.

"You don't just have cheese, you have good quality cheese and you don't just have bacon, you have qood quality bacon," says Rimmer. "With the burger you can do down the route of the joy of local produce and source meat and cheese from your local area."

Salads come from producers known to the restaurant and chutneys and mayonnaise are handmade. No matter how elaborate the additions, there is a constant.

"Burgers are all things to all people," Rimmer concludes.

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