Dfyjkt

I will fly from Reykjavik to Hong Kong and change flight in London.
I have Chinese passport and Iceland permanent residence.
Do I need an UK visa?

Ok, let me try..

You are coming to the UK to transit (on your way to somewhere else, Hong Kong in this case).

Have you bought your ticket as a single ticket from Iceland to China? If yes, it means that you will not go through border control.
So you will not leave the airport and will not have to check your bag again.

In a normal case, if you have a Chinese passport, and where in Iceland, going through the UK without passing border control, you will need a transit visa.

The transit visa is called a Direct Airside Transit visa (DATV) and it cost £34 (Sep 2017)

However,

In your case you have an Iceland residence permit. Iceland is part of the EEA (European Economic Area).

So you’re exempt: You don't need a visa.

For your information:

For e-visas and e-residence permits:

• E-visas or e-residence permits are not acceptable unless your airline is able to verify it with the issuing country. Contact your airline
  for more information.

How to know when you pass the border control:

• You always pass through border control if you:*


• leave the main airport building for any reason


• need to collect your bags and check them in to your onward flight

You must also pass through border control if both:

• Your onward flight leaves on a different calendar day to when you arrive


• There’s nowhere for you to stay overnight in the airport, for example in a transit hotel

All of this is based on your message and the info that I've got. Better to always ask the authorities about things like that.

The UK gov website is awesome, there is a questionnaire you can complete: gov.uk/check-uk-visa