You can get a lot if Gi GL friendly bars in the health food shops some of them can be a bit pricey.
Personally, I get Nakd bars which are very filling and don't spike the blood sugar. You can get them in tesco and health food shops. I also find the Nature Valley mixed berries bars good. Check out some of the aldi snack bars. Again a little if what you fancy is allowed. 75% cocoa chocolate is also allowed but just a few squares!!! As a choc treat.
Be careful of some of the supposed healthy ranges of bars as they have a lot of sugars in them to compensate for lack of fat.