It's not clear whether you're looking for a company in the UK organising such trips, or for a local tour company (in Regensberg, or possibly Munich) which runs them.
It's probably very unlikely that any UK firm will go there (unless as part of a much wider tour of that part of Germany). The Regensberg Touist Centre will know of any local companies organising tours to the BMW plant. You can email them at
[email protected]
Alternatively BMW will accept pre-booked individuals on some tours (which are mainly for groups). So you could take a flight to Munich and then hire a car. (Alternatively there's a direct coach service to Regensberg, from where the No.7 bus will take you to the BMW plant). BmiBaby fly to Munich from East Midlands, easyJet fly there from Edinburgh, Gatwick, Stansted and Manchester. Those airlines will probably offer the cheapest fares but Munich is also served by BA (from Heathrow), Singapore Airlines (from Manchester) and Lufthansa (from Birmingham, London City, Heathrow and Manchester).
Chris