UK MUMS!! Best formula?

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My baby is currently on Aptamil pro futura 1 and I’ve noticed he’s struggling to poo. Although his poo’s are not hard it’s making it really hard for him. He takes expressed breastmilk too as I had issues in hospital and had to stay. I’m trying to get him back exclusively on breast milk and build my supply but in the meantime, what are the best formulas for newborn?