The flagship financial index for the UK’s largest corporates (the blue chips) is the FTSE-100 share index. This index represents the UK PLC’s largest 100 companies.
Taking a closer look at the largest 10 companies that are in the FTSE-100 and seeing their representative percentage weight in the index gives even more insight:

HSBC Holdings = 7.69%
Vodafone = 5.59%
BP = 5.25%
GlaxoSmithKline = 4.77%
British American Tobacco = 4.12%
Royal Dutch Shell (A) = 4.01%
Royal Dutch Shell (B) = 3.56%
Diaego = 2.90%
AstraZeneca = 2.43%
BHP Biliton = 2.27%

[% weightings from Legal & General FTSE-100 Index Fund Managers Report]

Add up the percentages and you get 42.59%.

Yes, UK’s top 10 companies out of the 100 largest account for 42.59% of the FTSE-100.

The ‘remaining‘ 90 companies in the FTSE-100 such as BT, Barclays, Legal & General, Centrica, LloydsBankingGroup, Aviva, Prudential, British Land, Tesco, BG Group, Standard Chartered, National Grid, Arm Holdings, Marks & Spencer, Sainsburys, Severn Trent, Unilever, Burberry, Rio Tinto, GlencoreXstrata, The Royal Bank of Scotland, Land Securities, WPP, Standard Life, United Utilities, Anglo American, Rolls Royce,BAE Systems, Scottish & Southern Energy, RSA Insurance Group, Fresnillo, WM Morrison…. only account for 57.41% of the index.

Thus by investing into a FTSE-100 Index Tracker, over 13% of your investment is held in HSBC and Vodafone alone. The FTSE-100 is distorted by the UK’s  largest top 10 companies.