Londoners...can you shed any light on why

I'm not a Londoner, but I would imagine that it's because the Smoke wasn't nearly as diverse and multicultural in those days.

There's a scene in another Bob Hoskins film, "Mona Lisa," in which he arrives back in his old East End neighborhood to discover that all of the white English people have moved out and black people have moved in.

"Where the hell did they come from?!" he asks his old friend. "They live here," is the reply.

"Beethoven had his critics too, Keith. See if you can name three of 'em."

This was 1980s London and remember that Harold is an old school Londoner who probably wouldn't have associated with anyone black growing up or in the criminal circles he ran in.

it was the late 70's we did not mix on the streets let alone in gangs

I was a teenager when this film came out, i lived in East Ham so this is quite familiar to me. The film is set in london in the late seventies. there were very few immigrants at that time (excepting the irish). So that's one reason why.
The other reason though is there was barely any integration. so immigrants all went to the same areas. brixton was where all the black people who came to london lived (yes, i know a generalisation). As Harold controls the east end, there is no reason he would really come into contact with many blacks, as they mostly lived in south east london not east london - which therefore meant his mob would be, as from his area, mainly white.

Although its true that London was less diverse than it is now, it was still multicultural but filmmakers were less likely to reflect this.At the same time as this film was released so was Babylon that reflected the Afro-Caribbean community and racism in London.

Its that man again!!

Of course London was multi cultural at that time.
Music in particular brought black & white together.
Has nobody heard of the band UB40?
Nobody heard of Ska music?
It just wasn't AS multi cultural and diverse as it is now.
And I'm a black Jewish Irish man that grew up in London in the 70s and 80s.

The film was shot and set in 1979. There are black characters in the film (the mechanic on street and the Drug Dealer named Erol) a street in Brixton (a black neighbourhood) is depicted with virtually all black extras except for a couple of young white kids "minding Harold's car"! Harold also refers to black gangsters who deal in narcotics from another part of London (Tottenham) when he chats with Parkey on the docks.

Given the era, the story and characters the story is about, there would not have been any black faces in Harold's circle. The film got that right. I am certain though if this film was made now in 2015, but still set in 1979, then there would be black characters in Harold's circle (like in the film "Gangster Number One" which was made in 2000 but set in the 1960s, totally unrealistic).

I was told that Harold's base is suppose to be the Isle of Dogs. That area was still 99% White British in the time that this film was set. In the 1990s, when there was an increase in Asian families (something to do with relocation of council tenants from Whitechapel), there was a lot of racial tension and the election of a BNP councillor (who, when asked what he would do about clearing rubbish in the area, responded by saying that the Asians were the main "rubbish" in the area).

Even today, the Isle of Dogs is not as multi-cultural as the rest of London.