From my own experience typically aid stations are pretty good and plenty of the usual stuff that people are after will be there. If you are after anything really specific then the safest bet is to carry it yourself.
Common items to find at aid stations are things like mars bars, banana, flapjack, biscuits, jelly sweets, water, coke, orange juice. Some longer races have a designated aid station serving more substantial and hot food. Best bet to confirm any doubts would be contact the organiser. Having met the organiser for the Thames Path 100 I am sure you will get a helpful response.