I was just the same as you - completely hooked on sugar. I was getting through a chocolate cake on my own in two or three days and telling myself it was just because I was breastfeeding. It wasn't. I think it was due to insulin resistance in my case. I was eating the sugar cake, then my pancreas was pumping out loads of insulin to deal with the glucose, but then my blood sugar was dropping too low and I would get hungry again an hour or two later. And it wasn't just hungry, it was HAVE TO EAT RIGHT NOW hungry.
If something similar is happening with you then you may benefit from cutting down on carbs to get your blood glucose more stable. The problem is that all the carbs in your diet are broken down to sugar. You say that your diet would be relatively healthy but...
Breakfast- porridge - not too bad if you're making traditional porridge but if you're using something like Oat So Simple then it will have substantially more sugar.
cereal - Corn Flakes are 84% carbohydrate. A reasonable serving (i.e. not the miniscule 30g recommended serving size) contains around 50g of carbohydrate which will be broken down to sugar. Your blood should have around 5g of glucose so that one bowl of cereal is already massively overloading your body's system for dealing with glucose.
toast and jam/marmalade/ or sometimes just butter - one slice of toast with jam is adding another 25g of carbohydrate, some of which will be broken down to fructose rather than glucose but the fructose will be immediately stored as fat. Are you overweight at all?
yogurt - what type of yoghurt? If it's low fat then one pot could contain another 20g of carbohydrate.
You then list another four sugary snacks.
I would (very roughly) estimate that you're consuming around 400g of carbs, the average would be 300g and that's for a nation where 23% of the population are obese, not overweight but obese.
I tried increasing portions at lunch and supper, but I still wanted the sugar. - Your problem is not portion size, it is the sheer quantity of carbs/sugar that you are eating.
It's not that I get hungry, I just am addicted to the sweet taste and I get bored and snack... - Sugar is incredibly addictive. You may find it very difficult to cut down.
Maybe I should try replacing my sweet snacks with something savory but equally tasty, like crisps, to kick the sweet tooth and THEN cut down on snacks once I'm weaned off the sugar? - You definitely should cut down on the sweet snacks but be aware that a packet of crisps has around the same amount of carbs as two chocolate biscuits so switching to crisps may not help.
I'm biased because I eat low carb myself, but it was the only way for me to get my blood glucose under control. Trying to eat healthily just didn't cut it. I was eating cake, biscuits, crisps and loading up on bread and pasta which just break down to more sugar. When I went low carb the sugar addiction stopped within a week, it was completely driven by insulin. Losing weight was a fortunate side effect of going low carb for me. I work from home and in November I was sitting at my desk which was littered with my kids' Halloween sweets and chocolate and it annoyed me because it was in my way. It never even crossed my mind to eat any of it. My addiction to sugar was just that, an addiction. Now that I've broken the addiction then I don't need to eat sweet stuff and I don't get 'hangry' any more.