In message .uk, at
21:25:00 on Sat, 1 Mar 2008, Colin Rosenstiel
remarked:
http://www.railfaneurope.net/pix/ch/.../Geneve-04.jpg

That's the only one looking to be above what diesel technology could
deliver. I note that Geneva trolleybuses are all single deck, though.

A double-decker might foul the wires!

The only remaining modern double decker trolleybus may be this one in
Hong Kong (a trial, apparently now discontinued):

http://www.trolleybus.net/hk.htm

It's not clear how much bigger than a standard diesel double that is,

About the same size as the larger ones (as used secondhand as tour buses
in London).

let alone whether it would be able to work in the UK.

I don't know why it wouldn't, if the wires were a suitable height.
--
Roland Perry