I saw this on another forum a while back, can't recall where, so I cannot credit the clever soul who worked this out. Hopefully helpful to those looking for a used car. It used to be the case on the BMW AUC website that the VIN number was in the filename of the pdf download for each vehicle. Then you could check the build spec of the car using one of the online VIN decoders. However, the site no longer does pdfs with the VIN number in, so how to get from the reg no. to the VIN number? Give this a go... Go to: http://www.247spares.co.uk/ enter the reg number, then hit enter press CTRL_U press CTRL_F search for the letter VIN and after about three hits around line 207 will be the full VIN of that car. Simples...
Or you could just go to https://www.bmw-warranty.co.uk/Start, click on get a quote and then type in your reg. The next page shows the VIN.