Romford is another outer London area that has lost a lot of cross-border services over the past few decades.
Now, it just has the 370 to Thurrock (itself curtailed from Tilbury after it became part of the TfL network), the 375 into Epping Forest (itself the tiny remnant of far more extensive services that used to run along the same roads before continuing to either Abridge, Epping, Harlow or Ongar), and the 498 (like the 375, a recent TfL replacement for part of a much longer former NBC, in this case Eastern National service that used to continue beyond Brentwood to Chelmsford, Canvey,or even Southend). And I suppose the limited service 347 just sneaks over the border into Thurrock.
What has been lost since 1986, apart from the above (with the services to Harlow, Chelmsford, Basildon, Canvey and Southend being fairly substantial losses) also includes
... the service via the back roads (via Warley) to Brentwood that variously was part of the 247 and the 347, or following its diversion to Bulphan (which always seemed an unlikely destination to be linked with Romford) the 265.
...another service to Brentwood via an even more convoluted route via Upminster, West Horndon and Herongate, that was latterly the Eastern National 226
...another service to Basildon also via Upminster and West Horndon, the 403, which had been operated by both Eastern National and London Country at various times
...the "express" service to Southend (the 400 or the 2) via the A127 more or less direct (which used to run all the way from Wood Green or possibly even Kings Cross)
...the Thames Weald "Tunnel Express" minibus service (quite an eccentricity, in numerous regards) that went to Sevenoaks and various other places - essentially aimed at people coming to Romford Market from North Kent.
I think there were numerous reasons for these withdrawals, some commercial (it's hard to imagine the 226 or 403 having been viable, while clearly other retail destinations like Lakeside or Bluewater) supplanted Romford as a bigger retail destination, some to do with increasing car usage, but others were to do with franchising requirements and the unwillingness of local authorities (be it TfL or Essex County Council or Thurrock) to fund services that went far outside their areas. But the overall effect has been indeed to create a "Berlin wall" of bus services between Greater London and some of its surrounding counties and to discourage longer-distance journeys being made by bus. In some cases (eg Romford to Harlow, which used to attract an hourly service, seven days a week), some journeys are effectively impossible to make by bus.