Warwick Services is 17ml from the M42, and Cherwell Valley Services is 40ml. There's an Asda superstore near Junction 4 (approx 70ml from the M42), turn left on to the A4010.
You can get off at High Wycombe and follow the signs to superstore for Asda superstore petol station. Cheap for this area. Less than a mile round trip from junction.