im from the UK aswell mate,
i think your looking at the wrong yougurts. you need to be looking for the natural yogurt. as stated above i use the brand “rachels” but you can even get tesco own/ asda own brand it doesnt matter as long as its plain natural with no flavours like strawberry bannana etc…..
what does matter is the ingredients which should just be milk and thats it and thats why its allowed on ALL STAGES of the diet coz theres nothing else in it and definately no sugar. if you still cant find it look for greek yogurt and again go for a plain one with no flavour and youll be fine.