It is just after prime minister's questions, and it's all rather lively in the Strangers' Dining Room in the House of Commons. Sir Peter Tapsell, father of the house, is at a corner table, burbling contentedly. Tory and Labour MPs are rigidly segregated. A staff member with Charles Darwin's beard spoons out crumble and custard. Down the corridor in the empty bar they are serving "Top Totty Blonde Beer", with its bunny-eared model. By the following day this will be withdrawn, after a complaint from the shadow equalities minister, Kate Green.

I am here as a guest of MP Kerry McCarthy, having read recently of the appalling hardships our Honourable Members endure in their dining rooms and refectories. "Literally uneatable" was Tory MP Laurence Robertson's verdict on the food served in the Commons last year. Another member bewailed their "bucket" of chips, adding that while such presentation is "no doubt trendy", it makes the chips "soggy". ("The tower arrangement is better," this gourmet claimed.) Packets of crisps from Commons vending machines are 10g too light. The beetroot is "tasteless", the eggs are "watery" and the salads are "cold". In all, despairs one MP from the wood-panelled dining room with its sweeping views of the Thames, eating in the mother of parliaments is "a dismal experience".

There are, remarkably, 28 different food outlets in the Westminster complex. The grandest and most traditional are the adjacent Members' and Strangers' Dining Rooms. These share a menu, the former's being heavily subsidised. Only MPs and officers of the Commons are allowed in the Members', the Tea Room and various other places. "I don't like the food and can't eat most of it," says McCarthy, who is a vegan. "I think it's generally pretty OK - though some of the combinations are a bit bizarre." Starters at the Strangers' include rabbit and apricot terrine or roast partridge breast, both £6.75. I have chicken with cabbage and black pudding potato cake: tepid but tasty and, at £13.55, cheap compared with many central London restaurants.

In their private dining room, MPs enjoy the same dishes for a fraction of the price. On that menu last month, according to political blogger Paul Staines – AKA Guido Fawkes – a braised pork belly with black pudding bonbon and apple salad starter was £2.70, while a rib-eye steak with béarnaise and hand-cut chips (I trust these are now being stacked in "the tower arrangement") was generously priced at £7.80. Down the road, a lamb main course (though presumably more expensively sourced) at Roux at Parliament Square is £29.25, while decent steaks haven't cost eight quid in central London since we had a female prime minister.

These prices are only possible thanks to an annual taxpayer subsidy totalling nearly £6m, which has risen almost 20% since 2008 – despite a promise in 2010 to cut it by £500,000 and bring bar prices in line with the high street. For every £10 an MP spends on lunch, the public contributes £7.60.The £7.80 rib-eye in the Members' Dining Room carried a subsidy of £5.92: the taxes from several hours' work paid by someone on minimum wage.

Staines, who runs something of a campaign against the subsidy, tells me: "Three courses served by liveried waiters for the price of a KFC family bucket is rubbing our faces in it." In The Terrace restaurant, open to more or less anyone working full-time at Westminster, cumberland sausage with mash, yorkshire pudding and onion gravy cost £2.95 the day I visited – a cheap lunch, doubtless deserved by the cleaners, security staff and often poorly paid aides and researchers who work to keep the country running. But as Staines says: "The subsidy is greatest in the dining rooms for the exclusive use of MPs. The interns will pay double for the same meal served in a different dining room. What kind of legislature expects low-income, taxpaying voters to subsidise its boozing at all hours?"

• This article was amended on 21 February 2012 to removed a superfluous personal detail.