Well... the premise of your questions are wrong.
HK is not "revolting against" China per say.
And the UK is not "supporting" an "independent" HK, per say.
So, let's start with: what is HK to China.
When the British left HK in 1997 (also known as the Handover of HK, or "The Return" in China), they transferred the authority of the region to China which created the "Special Administrative Region of Hong Kong" with governing and economic systems separate from those of Mainland China.
That kind of Region benefits from special codified constitution, granting it laws providing a high degree of autonomy, a separate political system, and a capitalist economy.
HK operates under the "one country, two systems" constitutional principle meaning the Government of HK is, in law, exclusively in charge of Hong Kong's internal affairs and external affairs. The PRC of which the Hong Kong government is financially independent, is responsible for Hong Kong's defense and foreign policy.
Since 1997 there have been many political issues with HK and China's will to "absorb" that region, most of which stemming from electoral reforms and the interpretation of "Basic Law" (which more or less defines HK as an entity).
Up until 2019 when PRC tried to pass the Hong Kong Extradition Bill, to amend extradition rights between Hong Kong and other countries. This bill was proposed because of an incident in which a Hong Kong citizen killed his pregnant girlfriend on vacation in Taiwan. However, there is no agreement to extradite to Taiwan, so he was unable to be charged in Taiwan. The bill proposes a mechanism for transfers of fugitives not only for Taiwan, but also for Mainland China and Macau, which are not covered in the existing laws.
The 2019 Hong Kong District Council election was held on 24 November, the first poll since the beginning of the protests, and one that had been billed as a "referendum" on the government. More than 2.94 million votes were cast for a turnout rate of 71.2%, up from 1.45 million and 47% from the previous election. This was the highest turnout in Hong Kong's history, both in absolute numbers and in turnout rates. The pro-democratic party won by a landslide, their seat share increased from 30% to almost 88%, with a jump in vote share from 40% to 57%.
Anyway, while the whole covid thing taking place, on 18 June 2020, the Chinese government introduced a draft to the NPC, aiming for the session to take three days. According to the decision, the proposed national security law seeks to prevent external interference in Hong Kong's affairs, criminalize acts that threaten national security such as subversion and secession and allow the PRC to establish a national security agency in Hong Kong when China sees it as necessary.
In short: no more "one country two systems" principle, rule of law and civil liberties. But China said that the laws would only target a minority of "troublemakers" who had endangered Chinese national security.
HKers, obviously, don't want that. Thus the "revolt against" China.
The United Kingdom has historically been a popular destination for Hong Kong immigrants due to the colonial relationship between the two territories.
Etc...