There is no doubt some of the 40+ day matured steaks direct from the farmer mentioned here are going to be pretty amazing but the OPs asking how to recreate a £15 steak dinner at home for a reasonable cost.
You just need to shop around for a butcher you're happy with and also try a few different cuts. As for cooking it I'm sure you'll get 100 different suggestions here but for me I just rub a small amount of oil on each side of the steak (let the steak come up to room temperature from the fridge before cooking), a GOOD pinch of salt and pepper, then into a smoking, dry griddle pan for 3-31/2 minutes each side (based on around 1" thickness), and then let rest for 5 minutes before plating up.