Birchie, you remember correctly--I do not eat mackerel. But I am fond of smoked salmon, kippered salmon, or lox as part of a meal at any time of day. Cooked trout used to be a favorite family breakfast food from summers in my childhood. We were fisher folk. Sad to say, the trout filets were fried in bacon grease... But that's not a requirement, of course.
I like eggs, too. Eggs are a good food; don't believe anyone who says otherwise! And I mean whole eggs, not egg whites. Yuck.
Another good choice is 2% fat cottage cheese. Nicely balanced food! As good as meat, really. Sometimes we cook scrambled eggs with cottage cheese as a dinner item (or tea, as you might say).
Nuts are often a good choice, as long as you watch the fat amounts in other foods and compensate. I don't mean nut butters, although sometimes those can be OK. I prefer whole nuts. I like hot oatmeal with raw cashews (chopped) added just before serving and stirred in. A little milk added before eating is good, too.
If by Greek yoghurt you mean the 0% fat kind, I prefer some fat. Lowfat organic yoghurt is fine for me. I drink 1% milk--I personally wouldn't bother with skim. I've trained myself to the 1% over time, and now whole milk tastes like cream.
Hope this gives you some ideas.