Nothing to look for, if the new cans were in store they'd be online.
Go look in store and report back :) But maybe it is 440ml, not 375ml, I don't know as I'm not interested at all in the small can, all I know for sure is that it's smaller.
Looking on the big 4 supermarkets, none have 375ml. Asda have single and multipacks at 500ml. Tesco have single at 440ml and multi at 500ml. Sainsburys have 500ml single and multi and morrisons have 500ml multi with singles of the doctor at 500ml. If you couldn't find them in store then they wouldn't be on the website.
Well no because of the sugar tax.
website does probably as back-stock, but unlikely you will find the single cans in store... (I may be wrong, I'm basing this on working part time at Tesco myself)