You may already be aware of this, but
H188 OSG (188) from that small batch survives in preservation at the Scottish Vintage Bus Museum at Lathalmond. Not sure if it's able to carry passengers yet (not that it would've had much of a chance anyway within the last 13 or so months!), but as you can see, it's lookin' pretty bloomin' good now. All credit to her owners!
E: Darn,
@DunsBus beat me to it! lol
In a similar vein,
MDS 862V was the only National Greenway conversion ordered by a Scottish operator, specifically Kelvin Central as their 4138.
A few others that I don't think have been mentioned...
SWG 311S (South Yorkshire PTE's 511): the sole East Lancs-bodied Foden NC, the other 6 complete examples all being bodied by Northern Counties (hence the NC in the model name).
Greater Glasgow PTE AV1-3 &
AV4-18 (Alexander-bodied Volvo Ailsas). While Alexander-bodied Ailsas are far from unique, a few things made Glasgow's earilest 18 examples stand out from the rest. AV1-3, the pre-production trio, were the only Ailsas to have the panoramic windows in conjunction with the 'peaked dome' AV-Type bodywork. AV4-18 meanwhile, were perhaps the oddest Ailsas built aside from the single-deck Marshall example mentioned upthread and the low-height Derby example. As the A Type-bodied Atlantean was still Glasgow's bus type of choice at the time (mid-70s), these 15 Ailsas were specified and built with a 'LA'-style dome, no doubt for reasons of parts commonality. They were the only Ailsas built with this style of body and now-sole survivor (MGE 183P/AV8) is a gem among gems.