Sometimes, you need to fight fire with fire.
Psych yourself up, get your bag ready and then grab the items out of their hands as soon as they beep them over the machine. The moment there's the slightest of delays, bellow, "Come on. Come on!" and continue to grab the items until they're all beeped.
You need to already have money concealed in the hand holding the bag open, and before they have a chance to look at the screen and ask you for the total, thrust the money into their hands.
At this point they usually look quite startled, especially the Polish and other eastern European workers because they've never seen a Brit move so fast, not even on telly. They then get quite flustered trying to match your speed, getting the change and receipt ready.
Naturally you already have your open hand stuck right out, demanding the chance and receipt, which you snatch from them as soon as they get it close to your hand.
Finally, you politely thank them and shoot off like a rocket, making way for the next customer, who's invariably a slowcoach, only adding to the perception of your glory.
Repeat this several times and eventually the entire line-up of till staff will learn to both fear and respect you.
PS. Before Christmas, they had a batch of German egg pasta tagliatelle, 500g for £1.59. They came in a tall plastic bag with a label tied to the end with string, but I forget the name. It was delicious, despite having less egg in it than my normal pasta. Sadly, by the time I went back to stock up on them, they were sold out. But I'm sure they'll have another batch one day.