I was very sorry to hear about the demise of 'old bikey', lost I understand to one of London's bike devouring potholes. The passing of an old retainer is always a sad affair, but with cycles this sadness is always tempered by the knowledge that they can be readily replaced with something a little swifter, more eager and less querulous.
I understand that your new steed is to be flat barred and British. Patriotism immediately narrows the field quite considerably and of the many fine British marques we can immediately rule out the likes of Genesis, Orange, Condor, Kinesis, Dawes and Raleigh as either not building a suitable model or no longer being British owned.
One of the remaining options would be Chris Boardman's range of estimable bicycles. The hybrid models are sleek and swift and have more racing bicycle in their genes than is normally found in a commuting conveyance. Most of the models can be dismissed as being too skinny of tyre to truly suit the vicissitudes of a London road; only the more sturdy and dumpy MX model with its basic suspension fork and disc brakes looks to fit the bill. However practical it may be, easy on the eye it most definitely is not.
The nearest British equivalent to old bikey is the Charge Grater. With a lightweight aluminium frame, lots of gears and designed for both mudguard and rack this would be a worthy and practical if somewhat dull choice.
More interesting is the Cooper Casablanca created by a company better known for their forty year association with the Mini. Cooper are not shy about letting you know about this racing heritage; the cycle is apparently named after the racing circuit in Morocco where Cooper's F1 cars first raced in 1959. The cycle itself is designed with commuting in mind rather than racing, has eight hub gears, Reynolds (British) steel tubing, a Brooks (very British) saddle and even has a Union Flag (still British but whither the Saltire) emblazoned on the forks.
In spite of this prominent display of British heritage the Cooper, as with all of the bikes listed so far is course only partially British. While the conception may occur in the UK all are gestated and birthed in a factory in Taiwan. If your next bike is to be entirely British the choice becomes very narrow indeed.
One option would be to engage a custom frame builder. Bespoke bikes have been enjoying a renaissance in recent years with a plethora of builders both established and new to choose from; Mercian, Bob Jackson, Chas Roberts, Shand, Donhou and Swallow to name but a few. While the result would be a unique bicycle built to your exact specifications that would last a lifetime I suspect that an unsympathetic press would be quick to brand this an elitist choice.
Off the peg there are three marques still manufactured in Britain. The first is the resurgent Brompton. Whilst undeniably well designed and engineered, all folding cycles are a compromise and, unless forced to choose a folder by a lack of storage space or tyrannical train companies then you are wiser to look elsewhere.
The second is Moulton. The brainchild of the late Sir Alex, the Moulton may at first glance seem similar to the Brompton. In actual fact it is a very different beast designed to be a more efficient replacement to the traditional diamond frame rather than a compromised alternative to it. The Moulton has small wheels because they are stronger and lighter, a space frame because it is stiffer and full suspension to counter the bone rattling effect of small wheels and rigid frame. The result is a very capable bicycle that holds the speed record for upright cycles and which has been toured around the world multiple times. However, if a camera adds a couple of pounds being photographed on a small wheeled bike is likely to add a few more. People tending toward the robust can look a touch ursine on such a contraption; if considering a Moulton it would be wise to trial it under the watchful gaze of a critical friend before making a purchase.
The final option is Pashley. Manufactured in Stratford-upon-Avon since 1926 Pashley cycles have chosen the route of slow evolution rather than leading, pursuing or indeed taking any notice of new trends. Such is the cyclical nature of fashion that they are currently quite trendy in spite of themselves, their popularity bolstered by the wryly ironic Tweed Run. Nostalgia is however a dangerous constituency for a politician, and a Pashley is the vehicular equivalent of John Major's 'long shadows on cricket grounds'. For this reason I say avoid the über traditional Roadster; it is fusty and the type of bike that an old maid would select to convey her to communion on a misty day. Another model, the Gov'nor at first seems similarly anachronistic based as it is on a 1930's design. Possibly because it is based on a racing model it is however less stuffy than the Roadster and is in my opinion less staid than the Grater. Added to this the Pashley is supremely practical for modern roads beset from above by frosts and floods and from below by earthy eructations. The modern British road of pot, sink and probably kettle and hob holes is not so dissimilar to the unpaved roads the Gov'nor was originally designed to traverse. With its springy steel forks and heavy 28 inch wheels this is a bicycle that will sail serenely over all obstacles.
So there you have it, a small but eclectic selection of cycles awaits your critical eye. Choose wisely and take comfort from the fact that after (presumably) riding a Boris bike for the last couple of weeks any of the above will seem almost skittish.