Scan and go doesnt weigh items.
You scan the item and put in straight in your bag. You then take the scanner, to the scan and go area and their machine reads the scanner.
They have periodic quality checks. The algorithm, is quite complicated. You can be quality checked at anytime. They quality check by staff scanning items from your shopping until they add up to a certain amount. If there is anything missing they have ti scan the whole shop. They dont usually ban for a one off missing item.
They would only know if they quality checked you, which they didnt. Or if they were watching you on CCTV. Which is usually only done because they are suspicious. They have probably seen OP and suspected before, recognising her because she is there often.
OP, you shop lifted. Picking up multiple items and only scanning one is a common way people shoplifting using scan and go. No one in store or the company will believe you did this several times and just didnt realise.
I am also confused that you planned on paying cash. The majority if scan and go do not accept cash. If any do at all. It's a pain in the arse.
I am also unaware of a baby milk that only asda do.
You will need to order online and get bits inbetweeners shops from elsewhere.
I am more concerned you are scared to tell your dh. Whats going on there?