The UK’s benchmark index is the Financial Times Share Index 100, commonly known referred to as the FTSE-100.
Oddly enough 100 of the UK largest corporates are what make up the FTSE-100.

What is not widely known is the real distortion of the index by the 5 top UK company titans:

Shell, HSBC, Vodafone, BP and GlaxoSmithKline totally dominate the index. The size of these companies based on their market capitalisation explain the level of distortion of the FTSE-100.

Shell: [Shell A plc] £79,742m + [Shell B] £53,458m = £133,200m = £133 bn
HSBC: £123,609m = £123 bn
Vodafone: £111,879m = £111 bn
BP: £88,851m = £88 bn
GlaxoSmithKline: £77,995m = £77 bn

Total Value = £535,534 million = £535 Billion = £0.535 Trillion

However the % weighting of these 5 companies on the FTSE-100 shows how this £535 Billion skews the FTSE-100.

Shell: = 6.18 % of the FTSE-100
HSBC: = 6.17% of the FTSE-100
Vodafone: = 5.32% of the FTSE-100
BP: = 4.09% of the FTSE-100
GlaxoSmithKline = 3.79% of the FTSE-100

∑ (Shell: = 6.18 % + HSBC: = 6.17% + Vodafone: = 5.32% + BP: = 4.09% + GlaxoSmithKline = 3.79%) = 25.55% of the FTSE-100 = £535 Billion

So the top 5 corporates make up over 25% of the FTSE-100. (thus the ‘other’ 95 companies make up the remaining 75%.

Also with 2012 UK GDP = £1.495 Trillion and the value of the top 5 companies being worth £0.535 Trillion, then the value of Shell, HSBC, Vodafone, BP and GlaxoSmithKline equate to 35% of the annual GDP of the UK.

Useful answer if anyone asks the value of the UK’s top 5 companies.