The UKX or FTSE 100
broadly consists of the largest 100 qualifying UK companies by full market value. Many of these are internationally focused companies, however, so the index’s movements are a fairly weak indicator of how the UK economy is faring and are significantly affected by the exchange rates of the Pound. A better indication of the UK economy is the FTSE 250 Index, as it contains a smaller proportion of international companies.
FTSE 100 companies represent about 81% of the entire market capitalisation of the London Stock Exchange. Even though the FTSE All-Share Index is more comprehensive, the FTSE 100 is by far the most widely used UK stock market indicator. Other related indices are the FTSE 250 Index (which includes the next largest 250 companies after the FTSE 100), the FTSE 350 Index (which is the aggregation of the FTSE 100 and 250), FTSE SmallCap Index and the FTSE Fledgling Index. The FTSE All-Share aggregates the FTSE 100, FTSE 250 and FTSE SmallCap.