Thanks, that is a great price! You can check if it's the right oil for your car on Castrols site here https://applications.castrol.com/oilselector/en_gb/c/search
Correct the thinner the oil the easier to start at minus 30 and below. I do it every winter in Finland, minus 42 sometimes. Since we have had the 0w oils and better batteries life is a bit easier.
It really doesn't. It is the SAE rating for the oil viscosity at 'low' temperature (ie. below the normal operating temperature of the engine) - The lower the number, the thinner the oil. SAE 5W30 is suitable for use in ambient temperatures down to about -30C.
Yes if you are lucky enough to see these on shelf there and not the empty shelf like I do get at Tesco' s usually for things on good offers, Tesco online also says not available. I ordered 2 from Amazon with convenience of free home delivery instead of goose chase at Tesco's and most cars needs more than 4L so with over £20 order it's free delivery for all at Amazon don't have to be prime minister member at Amazon ;) Anyways both stores got same price so whichever is convenient to you fine either ways as at this price it's not to be missed. :)
0 w 30 is better for very cold below freezing temps. Volvo for example specs 0w for their cars needing A5 B5 grade. ) the zero means 0 deg.