My house
Seriously if J5 M40 is about the right place, there isn't a service station but there is a large Asda just off of the motor way (you can see it from the M40), might that be suitable?
There are no service stations from that junction to the M25!
This website gives details of them all, being near to home I've not visited them!
http://www.motorwayservices.info/list.php?by=map&road=M40
Hannah