We live in Devon, so welcome to the westcountry!
Camelford is a very nice area in north Cornwall, you are very close to lovely beaches and also close to the camel trail (if you are keen cycling/walking).
Anyway, back to your main question - a good stop off point is at Taunton Deane Services (between Taunton and Wellington in Somerset)...Ive heard several people mention to avoid Segdemoor Services at Bridgwater, Taunton Deane is further on down from Bridgwater.
We've found plenty of room to park easliy/pull in at Taunton Deane and you have the usual burger king/shops etc there.
Also there are some lengthy/long term road works near this area which gets especially busy on Friday afternoons (so avoid this time if you can)...but a stop off at the services would break up the roadworks.
From there on down you have junction 27 (tiverton) services which arent really any good (just a garage and a little chef) and then you have junction 28 (cullompton)which is again small services with not very good amenities.
your next stop after that would be junction 30 (exeter granada services)...LARGE BUSY services with Harry Ramsdens/burger king etc...this is accessed by coming off the motorway by a couple of roundabouts with traffic lights (multi lanes)so it can get busy at peak times, petrol/diesel is very expensive here!
Exeter Granada would be your last services on the M5 then I presume you are heading on the A30 towards Camelford?