This is the Leeds trolleybus proposal, following goverment rejection of a Leeds tram system. It is the only trolleybus solution being developed in Britain at the moment. (As a blogger comments elsewhere, a major city such as Leeds really shouldn't be reduced to merely trolleybuses.)
At Brent Cross, there is no prospect of trolleybuses providing a 'high-speed' transport service, beyond that provided by buses, given the lack of extra road space. An 'off-road' light rail system, given the existing available railway corridors, is the preferred solution of the 'BXC Coalition', and the capital expenditure would be justified by likely passenger levels.
Not that Hammerson or the London Borough of Barnet are interested.