A quick brain dump of cafes and shops that we know from recent trips (because that's all cyclists need to know about.....):
Coop for food - Coops are your friend! One on Barra, about 1/2 mile from ferry, Daliburgh (South Uist), Creagorry (Benbecula, the biggest shop), Solas (North Uist). It won't affect you but for future reference some are now open on Sunday afternoons!
Other shops - Eriskay village, Leverburgh (west of the ferry from Berneray), Lochmaddy, Berneray (with the Bistro), Maclennans supermarket Benbecula
Cafes we have used - Barra airport (definitely try to see the plane landing/taking off), Eriskay (small cafe combined with the shop), Kilbride Cafe at the camp site (South Uist, not far from Eriskay causeway), Kildonan (at the museum/heritage centre, we have also just stopped off to use the loos here!), Hebridean Jewellery coffee shop (South Uist at the north end, not on the main road but signed and worth a detour), Grimsay Kallinn shellfish (a detour from the main road between Benbecula and North Uist - Kallin have a shop and also at the old harbour a little sit in cafe), Taigh Chearsabhagh (arts centre in Lochmaddy), Lochmaddy hotel, Berneray Bistro (note the owner is also a fisherman so local crab sandwiches are often on the menu), Butty bus near the ferry at Leverburgh Harris (may have limited opening times). This is not an exhaustive list - we haven't been to everything on the Uists (yet!). Also things change - what was the Island Deli in Benbecula has apparently now reopened as Charlie's Bistro (and seems to get great reviews)
Also newish (but no use to YOAV if B&B is booked) there is now a very smart bunkhouse on Berneray called "John's Bunkhouse". We had a guided tour from the owners this summer and it's fabulous, with lots of thoughtful touches and an excellent drying room.
Have a great time.