Assuming all goes to plan there is a chance that we’ll be able to see it around 20 minutes later, low in the south-west, passing from west to east. Attached image shows the anticipated path, generated with HeavenSat, using orbital elements posted by Marco Langbroek.
The path shown is for Cheshire … the trajectory across the sky against the background stars will be higher if you are in southern England, lower the further north you are.
It should pass below the moon around 20:53:33 GMT, below Leo around 20:54:23 GMT and between Arcturus and Spica at 20:55:08 GMT.
The ISS will make a visible pass (as viewed from the UK) around 11 minutes prior the launch of the Crew Dragon – as viewed from Cheshire it will cross the disk of the moon at 20:22:08 GMT.
Being in a lower orbit (and hence having a faster orbital speed), the Crew Dragon will spend the next 19 or so hours “catching up” with the ISS – docking is scheduled for 15:39 GMT on Thu 28th May 2020.
Note all times are GMT … remember to add +1 Hour to convert to BST.