I find the ASDA station in Northolt, at the top of Church Road next to the MacDonalds on the access road to the A40 is usually cheapest - but not really en route to the M1. Depending on how far up the M1 you are heading, you could always go M40>M42>A42 and join it at Kegworth (junction 24).
Richard Greenhough ● 1494d