An interesting theory. As far as can be ascertained, Rover didn't seem to do batches of unusual spec cars. The line was set up to build cars to customer order, so that any extras were batched as part of the build kit. I strongly suspect that even VIP's didn't all go down the line together - rather that the build kits were all assembled as a batch, and then the cars inserted into production over the course of a few weeks. Plus I can't see Rover doing a "loaded" version of the base model, they'd be more inclined to do a super spec version of the top model eg the VIP.
Proving one way or another is going to be tricky as the factory records don't record the options specified, even when they are very major ones like Air Con. The only approach I can think of that might work would be to find a dealer with a big enough throughput (eg Henly's of London) that they see a significant chunk of output from any given week. Then you need to hope that they are old fashioned enough to have kept old records.
Chris