Motorway Service Areas are leased out to various operators, mostly companies who specialise in the subject and run dozens of similar sites. A few organisations just run one site. The areas tend to be larger and further apart than service areas in many other countries - they are set about 15-30 miles apart on most motorways and some major A roads.  The locations of service areas are marked on many of the main Road Atlas maps that cover the whole of the UK mainland.  There are various Road Atlas books available at any bookshop in the UK.

Be warned that food and drink is heavily over-priced at these places, and is frequently of indifferent quality.   Petrol is also more expensive than the average price in towns, but the difference is not that great, and petrol prices tend to vary quite a lot anyway from one place to another.

In the main building, there will usually be a fast food chain restaurant (Burger King, Wimpy or McDonalds) along with the company's own food outlet, providing more substantial food, and a branch of one of the main UK coffee shop chains.   A newsagent will also be available, offering newspapers, drinks, snacks, souvenirs, sweets and ice-creams. Increasingly, there are also small supermarkets (e.g. M&S Simply Food).  

All service areas provide toilets and baby changing facilities and many have a basic play area for children, and slot machines for adults.   There will be a petrol station, separate from the main part, providing petrol and diesel fuel, along with an air pump for tyres, water for the car engine and occasionally, a jet spray or car wash. Again, snacks and drinks are stocked here, along with in-car essentials.

A motel such as Travel Inn or Travelodge can be found at most service areas. Should you be travelling late at night and feel tired, these are a good option to get some shut-eye, prices per room per night are reasonable and special offers can be had in the low season, making them a good option to take a couple of days out and explore the local area.

The operators include

If you don't fancy experiencing the delights, http://www.5minutesaway.co.uk/ has details for hotels, food and petrol within 5 minutes of a motorway juction.