Public Libraries News has published interesting information on outsourced libraries in Devon. A comparison to Brent is not entirely fair, as Devon has people spread over a much bigger geographical area, and with a much less developed public transport network than North London.
Nonetheless a couple of comparisons strike me.
Devon has put all its libraries in a charitable trust. This has actually created inefficiencies with new managerial posts being created to mirror what used to be done by the Council itself. The Devon Libraries Unlimited now seems to have a management ratio of 1:2. Brent Council by contrast from quite early on in the 2010 Labour administration restructured to reduce management layers going from a ratio of about 1:2.6 to much closer to the 1:6 ratio that was the local government standard.
A second striking fact is that Devon is still boasting about "keeping libraries open" i.e. the physical buildings. It doesn't mention any measures of usage, and I suspect that might because the usage figures don't look good (unlike Brent).
Finally the savings that Devon identify sound more like rent seeking savings rather than actual ones. Essentially charitable status allows for inventive tax avoidance.