Is there any serious prospect of the Castlebar line being added to the Overground fold in 2019? Either when the franchise expires in 2020 or, as you suggest, even earlier.
Great Western will need to run diesel units on the other three Thames valley branches (Henley, Marlow, Windsor) for the foreseeable future, so it makes operational sense to use a common fleet for all four, rather than split one off to LO. LO would surely not want to have to maintain a one-of-a-kind train just for that branch.