If you have the Tesco app, there is a button to search for a product and it tells you if it is in stock. I had to do this yesterday (Manchester based) and there were a few stores with stock, just not my local!
Asda and Morrisons always seem to have a good stock of the first milk. Check them out if you haven’t already 😊
I think the only difference is the iron content, which from what I understand they can’t breakdown if under 6 months but if we’re talking a day or two before baby is 6 months I don’t think it will do any harm