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Generally the SouthWest, and especially Cornwall, has much higher rates of unemployment and less jobs available, hence the cheaper rates.
If your car is still under warranty I wouldn't consider going anywhere else other than a VW dealer. Like you, I also have zero mechanic skills and therefore stay away from DIY which also shifts legal responsibilities. There are some here who are mechanics/technicians/engineers who do this kind of work daily for a living and therefore are totally capable of DIY and saving themselves some money but they tend to forget that some of us have different skills instead.