From https://toomanyadapters.com/buying-sim-card-united-kingdom/ April 2019
Four operators: O2 (GiffGaff), EE (was Orange; is also Lycamobile, Lebara), Three, Vodafone (VOXI)
Best network: EE, then O2 and Vodafone. Three has least coverage.
He recommends EE for most travelers as it has better network outside major cities.
You can buy in airport but you'll have more choices and spend less money if you wait. Easy to find SIM card stores in central London; they'll discuss package options and install and activate the SIM in about 2 minutes. No need to show passport or other ID; you get a SMS notification confirming all is well and then it works straightaway.
From https://londoncheapo.com/technology/uk-sim-card-options-london/ June 2019
Their is WiFi all over.
|VOXI (Vodafone)|| 8GB $12.50|
|EE (Lycamobile, Lebara)||2GB $12.50|
|GiffGaff (O2)||3GB $12.50|
Questions to ask.