Here's a couple of "rare" types including probably one of the most famous one-offs:
London's future:
Queen Victoria Street. Sep 2006.
www.flickr.com
Arriva took this DAF SB220/Ikarus Polaris - was ita one off?
https://www.flickr.com/photos/190667959@N08/52020435824/in/photolist-2nfSkcW-2nMXbB8-2nNLh2J-2p1T9jc
BYD complete vehicle (only 2 in the UK?)
Millbank Apr 2014
www.flickr.com
If we are stretching the boundaries abroad, there were only 30 of these MAN/Berkhof double deckers that were originally ordered by Citybus but the order was cancelled and the majority went to KMB with one going to Kwoon Chung Motors:
Tsuen Mun, Hk 2007
www.flickr.com
The original E200 design including the rear door:
Cobham, spring gathering. Apr 2013
www.flickr.com
A couple more that come to mind...
The Northern Counties Foden double decker:
NorthWestVRT open day, Jun 2022
www.flickr.com
The Wright bodied hybrid single deckers supplied to Go Ahead London (13) and Travel London (5?)
North Greenwich, Feb 2009
www.flickr.com
Also, the Volvo B9 with Enviro 400 bodywork wasn't exactly popular with 3 in London and a handful with East Yorkshire. As far as I recall the only big order for that combo was Dublin Bus:
Kingston town centre, Feb 2022
www.flickr.com
D'Oiler Street, Dublin. May 2018
www.flickr.com