Quote from Lucio Buffalmano on April 9, 2020, 2:47 am

The news that Boris Johnson was moved to intensive care unit was shocking to some.

But, Trump being Trump, he used it for another of his power moves.
As per CNN, Trump said:

Trump: We have made tremendous progress on therapeutics I’ve asked two of the leading companies… I’ve asked them to contact London immediately (...) but they are ready to go

And from this video you can hear he then added:

Trump: These people, are the greatest in the world, and they're at the hospital

Offering to help with what Trump frames as the "latest technologies", it's like saying "we're the more advance big brother, we can take care of you, little brother".
That positions the US as the power-house, and the UK as the puppet friend who needed help from the US.
And of course, it also positions Trump as the man in power, able to send the best help wherever he wants (Trump only cares about presenting the US as the power house because that makes him more powerful).

The UK's reaction also underlines the power dynamics.
The UK's reaction was not so polite as some journalists titled, but more on the annoyed side.
Johnson's spokesman replied:

Any treatment [the prime minister] receives is a matter for his doctors. (=don't meddle in our affairs, and don't use us for your power moves)
On this specific point... We're confident the prime minister is receiving the best possible care from the National Health Service (=you're not any more advanced than we are)

Declining was the only sensible thing to do, also considering that Trump was offering to help with non-tested drugs, which is nonsense.

But it's interesting to note that the UK declined quite strongly, which felt like it was a reaction at least partially based on pride. Since Trump is still the most powerful man in the world, it would have also been a good option to decline with more tact.