Most roads in London lie within a single postcode district. Not many lie within two. Hardly any lie within three. I've found one that crosses four.
Most roads lie within a single postcode district because postcodes are explicitly related to postal delivery. It makes sense for a postal round sorted by hand, and worked on foot, to try to keep entire streets together wherever possible. For example the first mile of the Edgware Road forms the boundary between postcode districts W1, W2, NW1 and NW8, but the Royal Mail's dividing line deliberately passes to one side so that the whole stretch of road is entirely within W2. Postcode boundaries may wiggle quirkily in places, but don't generally split streets in two.
The only street I've managed to find which passes through as many as four postcode districts is a long one, and an old one, the Harrow Road. This kicks off from the aforementioned Edgware Road and heads west towards, unsurprisingly, Harrow. By the time it metamorphoses into Harlesden High Street its house numbers are already in the 1000s, but we don't need to go that far, indeed the 'four postcodes' bit all takes place between the 300s and 700s. It's not an especially exciting stretch, sorry, but it is an excuse to go for a 30 minute walk.
Harrow Road W2 [Paddington]
The road's first three quarters of a mile are overshadowed by the A40 flyover, and the postcode district doesn't change, so let's skip that. We'll join things as the road veers off, uphill, just beyond Royal Oak tube. This is a fairly lowly corner of the London borough of Westminster, an area called Westbourne Green, where council blocks and highrises are the order of things. Westminster Academy stands out as a rare 21st century addition. The parade of shops beneath Oldbury House boasts generic blue signs denoting what's sold within - Fish Bar, Dry Cleaners, Hair Salon - but only for businesses that haven't changed hands since they were installed. Although the local greenspace looks mildly attractive it was previously the site compound for the building of the Westway.
Across the canal things get older, on one side of the road at least, which is lined by four-storey early Victorian terraces with small shops on the ground floor. Most sell food, either to takeaway or eat in, with a definite emphasis on the Middle Eastern. Stop by for Lebanese cuisine, or a dash of Persian, or whatever it is the bearded chef with the jazz hands is beaming about. Many cafes have segregated space outside for a couple of tables, but all lying fallow until the weather picks up again in the spring. The Windsor Castle pub hosted some of the earliest gigs by the Rolling Stones, The Who and U2, but closed ten years ago and is finally being remodelled by a demolition crew. The shopping experienc peaks beyond Elgin Avenue, and is locally bustling, although an Iceland supermarket's about as highbrow as it gets. One of the shops is called Luxury Pound Plus, the commercial equivalent of having your cake and eating it. Lovers of plastic containers in various shapes and colours will find a wide variety to browse along the pavement.
The dividing line between W9 and W10 runs between Bravington Road and First Avenue, and unusually chops one short shopping parade in half. From the Fridge Centre up to the pharmacy at number 570 it's all W9, then nextdoor at 572 Ahmed's Halal Bazaar is in W10. The streetsign on the side of the Queen's Park Meeting Hall (now the All Stars Boxing Gym) confirms the intermediate change.
Harrow Road W10 [North Kensington]
If you've ever walked up the Grand Union towpath from Little Venice, this is the moment when the Harrow Road briefly appears across the other side. It can be accessed on foot via the Ha'penny Bridge. In November nobody risks sitting on the adjacent benches, but pigeons perch on the railings for a better view. The further west you walk the emptier the retail units become, until they all are, their former shop windows sequentially shrouded and sheeted. A further canal break occurs opposite Queens Park Public Library, or PVBLIC LIBRARY as the painted signage has it. Next come 1980s-style terraced flats, and what used to be The Flora (but no longer offers Cooled Lagers and a Patio Garden), and then W10's half mile is over.
Harrow Road's junction with Kilburn Lane and Ladbroke Grove is also where Westminster meets Kensington & Chelsea meets Brent.
Harrow Road NW10 [Willesden]
Unusually the south side of Harrow Road remains in W10 while the north side enters NW10. Only Brent have bothered to put a street sign up, confirming the latter. On the W10 side it's no time at all before Kensal Green Cemetery kicks in, a vast space whose residents have no need of postcodes, but they do have one and it's W10 4RA. Much of the old cemetery wall is being replaced, initially by temporary panels, but one scaffolded section further up retains a proper Kensington & Chelsea street sign. I was expecting this to say W10, but for some reason it says NW10, so I'm now wholly unsure how the postcode boundary hereabouts really works. In good news this doesn't matter, because we're already on postcode number four so there's no need to walk any further.