What methodologies are there for determining how much money tracks an index such as the FTSE100? Are there public estimates?
Some additional thoughts (that may be more work than you want to do):
- Regress fund returns on FTSE 100 index returns and look for $R^2$ above some cutoff.
- Same thing but take a Bayesian approach, assign probabilities of indexing.
- Download all prospectus and do textual analysis for phrases that indicate following the FTSE 100 index.
- You might be able to get a reasonable lower bound by manually checking assets under management (AUM) for some of the biggest passive fund companies: eg. Blackrock, Vanguard, etc... (I don't know what the big UK specific index players may be. HSBC?)
- Searching FTSE 100 on Morningstar may be a reasonable source of data?
Every mutual fund/ETF is expected to pay a licensing fee to the index provider for the benchmark it tracks. I don't know whether index providers make that data available.
Alternatively, you could screen mutual funds/ETFs by benchmark and aggregate the market capitalization of the results.