I believe the US represents itself as the greatest democracy in the world. Acting on this belief, it has been keen to influence how other countries are run and has even had a hand in destabilising leaders and governments. I therefore think that what happens in the US matters to the UK and to the rest of the world.
Aside from that, while I don't believe our police force and other public institutions are as blatantly racist as in the US, we too have significant issues on a number of levels, not least the disproportionate number of black people who have died in police custody or as a result of police actions.
I don't think the term "dog whistle politics" correctly describes the action of taking the knee:
"Dog-whistle politics is a form of political rhetoric in which coded language is used, thereby ensuring that a message reaches a target audience without making the general public aware of the specific content of the message. Much like a dog whistle, which is only audible to ears which can hear sounds in a certain frequency, dog-whistle politics often slides below the radar of the average citizen, allowing politicians to target certain groups of the electorate with very specific language." (Merriam-Webster)
Taking the knee is a simple action taken to demonstrate a commitment to anti-racism, and its meaning is not "coded" or "covert". I would describe Thatcher's "swamping" remark and Johnson's "letterbox" remark as dog whistles in that the they were a coded way of "othering" certain sections of the population in order to reinforce hostile attitudes and garner votes.