baby formula isn't the best for cats although far from the worst too.
I would stick with the cat milk (if fairly cheap from places like tesco) or even goats milk (doesn't have lactose in it) though to be perfectly honest it is only about 20-30% of cats who have a problem with cows milk... yes all cats will get an upset tummy if they have too much but if you look at size to milk ratio giving a cat a pint of milk is like you drinking 10 pints... I think you'd have an upset tummy too
My 2 moggy boys get a couple of egg cups of cows milk once in a while when I either don't have cats milk or whatever, roughly once a fortnight and never had an upset tum with them