The only British buses that can truly be held up as "iconic" are probably the RT and RM, considering their association with London and all things British. Didn't they ship a few Lodekkas abroad, paint them red and use them for tours in various places? Who cares as long as it's not the real thing as long as it's a half-cab double-decker with a rear platform entrance? Actually I'd probably say that the Lodekka in its various forms is probably iconic too, as it represented not-London in fairly large numbers.
I personally don't see the E400 as being iconic, but then I'm stuck in the past and anything beyond 1986 is modern rubbish!
Maybe it's too soon for such a modern vehicle to be considered a design classic. The same goes for the Wright's Millennium range mentioned in Jordan's post above. I suppose they were striking designs compared to what went before. The ALX400 to me seems more like a development of the R-type (and brief Royale successor). Slightly modified front end, and designed for low-floor chassis. I liked its "retro" look, but I wouldn't say it was iconic.
A few personal "icons":
- Alexander AL. Build on Atlantean and Fleetline chassis, they were the standard bus for three of the Scottish municipal fleets (Glasgow, Edinburgh, Aberdeen), and fairly short-lived in the fourth (Dundee), as well as a few fleets south of the border (Tyne & Wear and Portsmouth? spring to mind).
- Alexander AY/AYS - the archetypal Scottish single-decker, also found in a few English fleets
- NCME's double-deck bodywork as found on Manchester's fleet on numerous different chassis
- The ECW/Olympian combination. Probably not so much iconic as ubiquitous.
To address a few others mentioned above:
- Dennis Drat. I hated them with a passion, but yes they probably do have their place in history. Lightweight, small rear engine, 40-ish seating capacity. Oh, wait - the Albion Viking did that too, back in its day (I'll see your poxy Allison automatic and raise you a 6-speed crash box!)
- Optare Solo. This shouldn't be mentioned without talking about the MCW/Optare Metrorider that went before. The Metrorider was a purpose-built minibus while everything else was a van conversion - Transits, Mercs and (is it possible for buttocks to have memory?) the Dodgy S56s. The Solo was the logical successor to the Metrorider, and when they replaced the 709Ds on my local route it was a major improvement.
- Optare Spectra. Is it a Metrobus MkX, or isn't it? When I first saw it in Buses magazine in Reading Buses livery I thought it was stunning.
It's all very subjective, and there are so many "icons" that I could come up with. There is one that hasn't had a mention so far: the Volvo B10M. Bus, coach, single-deck, double-deck - it was up for the job.