The first one that comes to my mind is Roadchef's own venture into cafes, Fresh Food Cafe, there are only 10 up and down the UK motorways but these cafes are one of few that cater very well to Coeliacs with their own allergy menu and they seem to understand cross contamination, at least the one we went to did. They do a range or wraps, sandwiches and main meals as well as the odd snacks. You can find a Fresh Food Cafe at these services:-
Clacket Lane M25 (between J5 and J6)
Northampton M1 (J15a)
Norton Canes M6 Toll (between JT6 and JT7)
Rownhams M27 (between J3 and J4 Westbound only)
Sandbach M6 (between J16 and J17 Northbound only)
Strensham M5 (between J7 and J8 Northbound Only)
Taunton Deane M5 (between J25 and J26 Southbound Only)
Tibshelf M1 (between J28 and J29)
Watford Gap M1 (between J16 and J17)
You can read our review of Tibshelf Fresh Food Cafe here. We found them very helpful when we went and it has a better choice that most Motorway outlets can offer, so if you are near one we can highly recommend.
If you are not near a Fresh Food Cafe there are some other options, most services have either a Costa or a Starbucks, now they don't have much choice but if you are desperate there is at least something. Costa usually have a wrap, as do Starbucks, however if you are late in the day you will find it virtually impossible to get one as they don't tend to stock many. Costa have a choice of cakes for a quick snack, these are always by the till wrapped, Starbucks will usually have brownies and if you ask they will give you one wrapped. When it comes to drinks the majority at both are gluten free, but there is a few to be weary of, hot chocolate mostly. Take a look at this blog, if you want a bit more info about these outlets.
Waitrose and M&S have also started popping up in many services, honestly, they are small outlets and don't do much of the gluten free range that the bigger stores offer. We have found in some of the M&S's that you can get their Made Without Wheat sandwiches with the regular take away sandwiches. Waitrose tend to do a few bits of their GF range but it is usually limited to rice cakes and a few odd bits though. We did get a salad in Waitrose though at a few services so that at least is an option. Always beware that they are small outlets on the services and don't offer many options and tend to run out fast.
If you are very lucky, check in WH Smiths, they have been known to stock the odd gluten free Urban Eats wrap, but it's rare, worth a check though if you are starving.
If you are travelling to Cumbria you might have a bit of luck at Tebay Services, (J38 of the M6) they have a little farm shop which has some gluten free available, it also has a cafe which doesn't offer much but does usually have a cake available. If you want breakfast they do have an option or two, and for lunch there is soup and other options available, but remember they will be catering to the majority, which unfortunately for us is people without dietary issues, so remember they may not have too much to offer.
Ed's Diner are due to open 3 outlets on motorways soon, M1 at Peterborough, M1/M25 at South Mimms and A14 at Cambridge. Ed's have a full gluten free menu with burgers and hotdogs and are fab in terms of cross contamination knowledge.
Unfortunately many times I have had to take a pack lunch in case I couldn't get anything, so this is probably the best way to go if you know you are going on a long journey, of course you could strike gold and find somewhere suitable to eat but always go prepared!
If you know of anywhere else you can get some food on the motorway feel free to drop a comment!