Canteen looks so much the part. The owners stress that they take design as seriously as they do food. This new Baker Street branch is the third to be opened, the first being in Spitalfields and the second around the back of the Royal Festival Hall. They have in common a definite Festival of Britain look — if anything, more plushly done than the real thing. In a little promotional booklet placed on every table it states that the design is inspired by classless, communal, public spaces “such as schools, libraries, town halls and interiors” from that epoch.

It’s an odd ambition, to want your restaurant to express that welfare ethic from just after the war. But Canteen makes it look good. These are bright, open rooms, full of carefully chosen and commissioned modern furniture. Here at Baker Street, for example, there are lots of lovely Bauhaus-style pearlescent reading lights near the tables. The low tables, veneered in oak, are arranged in comfortable booths in a way that makes them feel both private and communal. It’s a thoroughly pleasing environment, projecting rational comfort, quality without ostentation, and it has clearly attracted a design-conscious clientele.

Canteen’s approach to food is similarly ethically based — carefully provenanced, additive-free, seasonal, the meat free-range, the fish landed from small day-boats on the south coast, the cheese sourced from Neal’s Yard, beer from the Greenwich microbrewery Meantime... Moreover, all the cooking, down to the jams and picalillis, is done in the kitchens, unlike, say, cheapskate Leon, where it’s bussed in from a factory in Hayes.

So that’s all great. On the other hand, the cooking style here is unremittingly British. We are supposed now to be lovingly rediscovering the lost greatness of British regional food at its best. Canteen, alas, often serves as a powerful reminder of how it fell into such disrepute in the first place.

At the Royal Festival Hall Canteen last week, smoked haddock, spinach and mash (£11.50) was a good piece of salty, undyed fish, perhaps a bit fiercely cooked and served with an overthickened hollandaise but enjoyable nonetheless, with some passable vegetables on the side. Successful assemblage rather than cooking but perfect comfort food nonetheless.

However, the stew of the day (also £11.50) — there’s always a stew, a pie and a roast — was hopeless. Billed as a chicken stew with thyme, it was horribly thin and watery, more a failed soup. Almost weirdly without flavour, it would have disgraced any home kitchen. A side-order of mushy peas (£3.50) was close to inedible: heavy, dark-coloured, with a truly off-putting taste. Another of duck-fat roasted potatoes was no good either — big lumpy tatties without any crispness or appeal.

At Canteen, as part of the nostalgia shtick, they put bottles of ketchup and brown sauce on the table. This food was an unwelcome reminder that, historically, much British cooking could be made to taste of anything only by being doused in such coarse stimulants.

In Canteen Baker Street, one of the group’s signature dishes, devilled kidneys on toast (£7.25) was much better — a generous seven halves of veal kidney, nicely cooked, on a slice of crusty brown bread, smothered in a creamy sauce, well spiced with cayenne, Worcester sauce and dry mustard and flecked with parsley. It would be a great pick-me-up, although a bit overpowering as a starter.

But another stew — beef, this time — was again beyond bland. The chunks of meat were tender and of obviously good quality but stewed with little more than carrots and celery, accompanied by dumplings, plus two big doorstops of bread and butter. Where were the things that make a beef casserole delicious: wine, shallots, bacon, mushrooms, say?

Another side-dish of “seasonal greens” was no more than sprout tops, steamed and buttered. Honest, full of goodness, unashamedly itself, yes. A pleasure to eat, no. British cooking of old was not necessarily unfairly traduced, re-acquainting yourself with it at Canteen obliges you to realise.

Somerset Maugham said to eat well in Britain you should have breakfast three times a day. Canteen certainly delivers great breakfasts all day (and, by the way, it has free broadband and plugs under the table, making it the classiest work station in town). And you could eat lunch and dinner enjoyably, too, if you ordered carefully, sticking to the straight and narrrow — say, the fine fish and chips.

The drinking is good, with a short well-chosen wine list opening at £13 a bottle, as well as an impressive choice of regional beers and ciders. Service is friendly and bright and the whole operation clearly has a good sense of morale and purpose. What it needs now is an overbearing French chef.