New answers tagged index
You need to read up on how the FTSE is calculated. See this link: http://www.ftse.com/Indices/UK_Indices/Downloads/uk_calculation.pdf. It involves the market value of companies, not simply a weighted price average. There is a detailed example in the document I linked, which happens to be the very first Google result for "ftse index calculation".
Usually the index provider has such historical data on its website. For FTSE see here
Top 50 recent answers are included