FTSE AIM UK 50 Index

The FTSE AIM UK 50 Index index was introduced on 16 May 2005, and is a market capitalisation weighted stock market index incorporating the largest 50 UK companies by capitalisation which have their primary listing on the Alternative Investment Market.

The index is reviewed quarterly, and the constituent companies may change based on market capitalisation data as at the end of February, May, August and November.

Current constituents of the AIM UK 50 index

...
Wikipedia