Most of the heroin comes from Turkish criminal groups. Liverpool groups mainly import it directly from Turkey. From Liverpool they distribute it to other cities in the UK (sometimes even as far away as Holland and Spain). In some places the Scousers both control the wholesale as well as the street-level sale. Some heroin also comes from Irish groups who themselves import it from Pakistan.

In London it's different. In North London there are already settled Turkish/Kurdish crime families (mostly Kurdish) who locally control both the wholesale and the street-level sale. In some other places there are Indians and Sri Lankans (mainly in West London) or Pakistanis (in several parts of the East) that are able to import heroin using their family connections. Again: both wholesale and street-level sale. Bangladeshis in East London are most of the time street-level drug dealers. In South London/Essex/Kent there are also local British criminals who buy it from the Turks/Kurds and from there on distribute it throughout other areas/countries (Spain is again a popular destination). In London's Southeast there are also Turkish Cypriots involved in the heroin trade, much of which is directly imported from Turkey. London's heroin trade is extremely colourful lol.

While local White British criminal groups are involved in the heroin trade to some degree, cocaine and hashish are much more important for them. Two places are critical for this: Liverpool and Amsterdam. In Liverpool cocaine is brought in from Colombia, while hashish is imported from Morocco. From there on you deal with the Scousers. The closest business partners of the Scousers are most probably the Glasgow based groups who then distribute it throughout Scotland and other places (there we go again: Spain ). Other smaller groups in the north and some in the midlands and the south buy it off the Scousers.
Larger London-based criminals have also used Holland a lot. Amsterdam has a shitload -and I do mean a shit fucking load- criminal organizations that import cocaine: you've got the native Dutch groups (sometimes connected to biker gangs like Hells Angels), settled Colombian criminals, Irish or Scouse criminals that settled in Amsterdam,... And if Amsterdam weren't enough the second biggest drug port in Holland is Rotterdam where cocaine and hashish gets imported by local Dutch, Moroccan, Curaçaoan and Moluccan criminals. London/Essex/Kent groups have a tendency to use small ferries to transport it from Holland to small ports in the South of England.

A lot of the local cannabis farms in South England are ran by Vietnamese criminals btw. Don't think the Triads have a hand in that. Triads, Albanians and Russians are a lot more involved in prostitution, human trafficking and weapons trafficking than they are in drug trafficking. Even though they sometimes dabble in that as well.