Yes, bread and grains/cereal are known gas producers. Nothing I have tried completely eliminates this. For this reason, I don't eat them. I suppose you could eat them when you know you will be home and the gas doesn't hurt much. I irrigate and as such completely forget about my ostomy. However, a little bread and I have to worry about "fart noises" when I'm in a social situation...it just isn't worth it to me.
Breads hold a lot of air by their nature. You can probably reduce some of the gas by thoroughly chewing (32+ chews before you swallow). That should break all the "bubbles" in the bread. Also, if you can limit your fluid intake to 4 oz during the meal.
New at this, I have found that both Gas-X and Beano help alleviate gas "blow-up", and both are available at Walmart online and Amazon. Don't know how that works for you over in the UK, but I'm pretty sure they're both pretty universal.... Good luck
Bread will cause my bag to balloon as well, but due to SBS, I'm limited to what I should be eating and bread is allowed. So I try to keep bread to a minimum when dining out, and I cannot go to sleep right after I eat it to avoid the dreaded blowout. I found this out the hard way after I ate stove top stuffing and fell asleep LOL! Thank goodness for bed pads :)
Irrigation is a flushing of the colon. It is possible, with the correct diet, to go 2 full days without any output. It eliminates leaks, irritated skin, and other problems associated with a colostomy. Not everyone qualifies, as you must have some descending colon left and it must be healthy. Most cancer survivors qualify. I suggest that if you have a colostomy, you ask your doctor or nurse if you are a candidate for irrigation. It's a life changer!