The only time I saw a Cambrian V806 at LBA as a reggie collector was in 1975. A footy charter to/from Paris in connection with Leeds United's ill fated European Cup Final. There may have been others ad-hoc or on CI flights but not routinely or for maintenance.
Vague recollection of the odd one pre-spotting days, perhaps 69/70, but only noticed 'cos it was 'different' in a red variation of NE's livery.
The order changed in April 76 when BA merged Cambrian/Northeast with Channel and Scottish operations to form BA Regional. Some V802s were withdrawn then including at least one that was scrapped for spares at Leeds. Thereafter operation was a mix of 802 and 806 variants.
I've an idea the 806 variant was built with more powerful Dart engines for certain hot/high or otherwise operationally difficult BEA duties. They were changed to the same version as the 802 when the machines passed to Cambrian/NE, becoming the 806x as mentioned above. Might have been some other detail differences in internal layout too.