With regards to starting early on Friday, there was a concert in the dome Friday night which is perhaps why they wouldn't want to shut the Jubilee early to get the engineers trains over to the Bakerloo. As it was the 11 trains all came over without any major problems, just a very slight delay to the Jubilee starting up again.
No physical barrier at BST that I'm aware of as there's no work happening on the Bakerloo in the immediate area. The points from the Jubilee to the Bakerloo are of course secured to protect the possession.
In addition to the works in the Paddington area (Warwick Avenue - 400m south of Paddington) replacing points and associated track renewing works there are activities down the southern end too. These activities are Hand-Grinding Works - Lambeth North 6A, 6B, 7A and 7B points. and E&C 26A, 26B, 27A and 27B points plus 8A and 8B points presumably Elephant too.
How long these activities at the south of the line will take who knows perhaps someone does ,perhaps this will block access to the London Road Depot and is an additional factor in deciding to close the entire line or at least make the Elephant - Piccadilly Circus service impossible not just operationally difficult.