It is easier and cheaper to purchase a SIM card locally and top it up with credit. There are shops in the airport that sell a variety of network operator SIM cards.
You could have free WiFi access in most of the major airports in the UK. The free access ranges from 30 mins to unlimited, depending on the different airport. Most towns and cities in the UK also offer some form of public access to the internet, and free Internet access is often available at libraries.
Heathrow Airport - wifi guide
Gatwick Airport - wifi guide
Manchester Airport - wifi guide