Many chains are coming round to providing gluten free options, some more reliable than others. My partner has Coeliacs, so is very sensitive and can be caught out by cross contamination from gluten free food being prepared in the same space as "normal" food. As well as the places mentioned above, Bella Italia and Whetherpoons can both provide gluten free, although we have had contamination issues with Whetherspoons. McDonald's chips are also gluten free if you need a quick pick me up.
Most supermarkets have free from sections which stock basics like bread, pasta and flour alternatives, as well as cereal bar type things (
Nakd bars are good for cycling snacks), just be careful as some things claim to be gluten free, but actually contain oats which have their own form of gluten that can cause some people problems (these products generally have warnings about not being suitable for Coeliacs).
The other good thing to do it just a general internet search for pubs and cafes in places you are likely to stop, they are becoming more common. All food businesses should also be able to provide with a list of all allergens within their products, so just ask and they should be able to find something for you to eat.