Bacon.
Good quality sausages (ones not filled out with breadcrumbs).
Or, variations on eggs.
I like egg muffins which are like mini omelettes baked in the oven in muffin cases which I fill with whatever veg I have in the cupboard (mushrooms, courgettes, peppers, spring onions, spinach, ...), cheese and often chilli sauce like Piri piri sauce.
I chop all the ingredients up into small cubes and beat into some egg. Pour into muffin cases. Bake for 20 minutes on a moderate oven (gas mark 4).
These can be eaten straight away, cold the next day or heated up in the microwave.
Alternatively, you could try yoghurt with a few berries.
This is not as proteiny as eggs and includes some carbs but if you chose something like full fat Greek Yoghurt and add your own berries, it is low carb.