The UK’s Flagship Index is the FTSE-100 Index.

The symbol is UKX

http://www.shareshop.hsbc.co.uk/shareshop/security.cgi?csi=50058

The top 100 UK companies but 12 companies distort the index due to relative weightings.

Royal Dutch Shell is 9.2% of the FTSE-100
HSBC 6.9% of the FTSE-100
British American Tobacco 5.4% of the FTSE-100
BP 4.7% of the FTSE-100
GlaxoSmithKline 4.2% of the FTSE-100
AstraZeneca 3.2% of the FTSE-100
Diageo 3.1% of the FTSE-100
Vodafone 2.9% of the FTSE-100
Unilever 2.6% of the FTSE-100
Reckitt Benckiser 2.5% of the FTSE-100
Lloyds Banking Group 2.4% of the FTSE-100
Prudential 2.4% of the FTSE-100

That is 12 companies make up 49.5% of the whole index.

What does this mean ?

So if you invested £1,000 with a FTSE 100 tracker, 49.5% of that £1,000 which is nearly half of your investment just goes into 12 stocks. That means, £495 of your cash is invested in the 12 companies above, and then the remaining £505 will be spread between 88 companies.
Thus your fortunes are tied to really 12 companies and their performance.  A very distorted index.