Thanks for sharing your experience. I know in the scheme of of things your repairs might not be a big deal, but you purcahsed extended warranty in good faith and you are entitled to get it it sorted.
Your story sounds that their argument was hardly based on fact but more so based on emotion and something more personal
?? I really could not believe their attitude when reading your post. Oil changes at a non BMW dealer causing electrical faults; using extortion tactics as leverage to coax you into buying a new vehicle from them. What the hell has buying a new vehicle from them have to do, with the problems you are having with your car now, that is covered by a current warranty sold by them. WTF??
As you know the BMW branded new car warranty is offered by Allianz which is a 3rd party service provider in this case. There should be no reason you can't go to another dealer or mechanic for that matter. An inedependant mechanic should be able to deal with Allianz. If you read the Extended Warranty PDS, I am sure it says you can take the car to an authorised repairer. An "authorised repairer" if I am not mistaken is a repairer that has a license to carry out motor repairs, in other words a licensed mechanic.
Give Allianz another call. Tell them what Doncaster BMW has said to you about their warranty and that you do not feel comfortable taking the car there for repairs. Ask them to recommend what course of action you should take. Maybe suggest if you can take the car to SouthernBM to undertake the repairs.
The Facebook suggestion sounds like a good idea. It might help with elevating your problem with BMWAG. Nothing like private issues with a customer being broadcast on the www to persuade them into taking some action. As much as you might hate to do that, it seems like it gets results these days. Keep us posted on how you get on.