I can't recall who built the GM Dealersport rally Astras- though as you say, I'm sure MSD in Milton Keynes were involved with Vauxhall at that time?
Safety Devices were definitely part of Vauxhall's rally programmes in the 80s for a while-as I remember they ran Pentti Airikkala's British Telecom Astra
http://www.flickr.com/photos/29877211@N08/5058022592/
After that though they moved on to run Pentti's Group A Starion in 1988
http://www.flickr.com/photos/daffy-dub/5867985364/
The name of Tim Ashton (BTR Preparations) also came to mind, as does Sydney Meeke in Northern Ireland, (he prepped Bertie Fisher's Manta 400), but I think they may have mainly been involved in some of the overseas Astra/Kadett programmes- Sweden, Belgium, Spain, etc
http://www.btrprep.com/btr-history