Its so annoying to be bombing along a quiet motorway at 80mph in the 1st lane, only to have to move across all 3 lanes to overtake some idiot in lane 2. Then you move back into lane one and he starts flashing you?
There is no actual law preventing overtaking on the left. The police don't always look too kindly on it and may try to get you for due care and attention but the case law shows that if there is enough room to safely undertake, ie. with a proper stopping distance allowed before you end up pulling out on anyone (which isn't even an issue if it's one car on an otherwise empty motorway) then the person who should be done for due care and attention is the MLM, not the undertaker.
Obviously if you undertake the middle lane person then immediately pull out in front of them because there is a truck in L1, or if you're weaving into the middle lane from L3 to undertake someone who is leaving too large a stopping distance for your tastes then they'll get you for due care and attention or worse.
As for the truck vs rail thing, what makes you think that rail freight is any more reliable than passenger rail? The in thing for supermarkets right now is the "just in time" delivery system where they don't keep huge amounts of inventory in stock (apparently this costs money, personally I think it costs the same but makes the books look worse, but then I'm not an accountant or a warehouse manager so I wouldn't really know).
In any case, the goods have to arrive within a very narrow delivery window, drivers are penalised for being early as well as being late which is why you always see Tesco lorries parked up blocking the street 5 minutes away from Tesco. Their stock system assumes the lorry will be doing 50mph to account for delays, so many drivers actually do 50mph to avoid the early arrival penalties causing the "elephant racing" long overtakes we're oh so familiar with (though to be fair, slight variances in the 56mph speed limiter are the causes of the worst of these)
You can criticise this system if you want. I'm certainly not fond of it, but can you imagine trying to make it work via rail? First whiff of snow or falling leaves and all the supermarket shelves would become empty. Even if they managed to invent some magical train designed to cope with Autumn and Winter, once it reached the depot you'd still need to load it onto trucks for the last leg of the journey unless you plan on building railway lines to every supermarket in the country, not to mention every business park, factory and every house that ever mail orders stuff from OcUK and the like.