The FTSE 100 is easily the most widely followed index in Britain.
It is an index of the 100 largest London listed companies. This includes many foreign companies but it does not include companies that only have secondary listings in London.
It is popular partly because it is familiar to investors. The fact that it has been available for many years means that there is sufficient historical data for most purposes.
Because the FTSE 100 covers about 80% of UK market cap there is rarely much difference between it and a wider index such as the FTSE-All share.
It is also easily available through many sources and published in real time.