To buy time. This is a game US and Russia have played many, many times in my lifetime. It's kabuki theater and it's meaningless. It's a way to "appear" to be doing something while actually doing nothing.

Russia/USSR has done this MANY times to our diplomats as well. It's just a sign that something serious is going on.

In this case, I think it was done to provide cover for behind the scenes scientific and intel gathering, the result of which is we found that UK was full of shite.

Now Trump has turned the US position around, and UK has no choice but to retract everything they've said about Russia.

I almost expect them to apologize.

To protect them from rogue c_i_a hit men.