There is little sense in having crews and aircraft overnighting in the UK when you have your own airline which can operate the routes. On LHR-VIE for example OS operate the first flight to London, while BD operate the first flight out of LHR.
Yes and no. From what I have heard, the idea is more to boost BD's performance by providing work for them without any commercial risk. This type of work is not new for BD as they have already based, e.g., an A320 away from home in CGN to serve the LHR route on behalf of Lufthansa.
Wasn't there also talk about BD taking over some more of LH's routes from LHR?