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The British petrol station (or 'garage') represents a quite perverse national institution that is well embedded in the psyche of the general public.
The Shop and the Fuel
Petrol stations in the UK normally have a small shop to go with them which sells snacks, bread and milk, and a few other basics - almost anything that you might need in a hurry. The prices of products sold in the shop are generally a bit more expensive than in other retail outlets1. They also sell charcoal.
The fuel sold in petrol stations comes in three varieties: diesel (derv), unleaded (ULG), and 4-star (LRP). There are several different types of petrol station to be found in the UK. The most common is the kind where you pay after filling up, although there are in some cases ones where you can pay at the pump with a credit/debit card, and occasionally ones where you have to pre-pay. Different types can confuse people not used to each system, so it is usually a good idea to make an educated guess from the surrounding area before risking potential humiliation.
Don't Try This at Home
It is illegal to leave without paying from a petrol station in the UK (a 'drive-off' or 'burn-off'). If you were thinking about doing so it may be worth noting that practically every petrol station uses CCTV cameras, and some won't even let you start filling the tank until they've made a note of your registration plates.
If you are frequenting your local garage after dark, or in the early hours of the morning then it is more than likely that you will have to pass goods and tender through a highly complex device stuck on the side of the building, so be prepared to suffer minor injuries to your fingers, or indeed anything else you try to stick through. If you have a large number of items to purchase then it a good idea to make a list beforehand. To save time and hassle you can hand it to the assistant who can retrieve your items from inside the shop.
The assistants are generally friendly and some do get especially lonely in the early hours of the morning, so they are always handy for a little chat, and most are familiar with local geography and customs, so provide a useful local information service.
Some petrol stations also provide either automated or manual car washing facilities, vacuuming, air pumps for tyres, and a water hose, although it is not recommended that you drink the water yourself even though it may be provided free of charge.