It swings from time to time. Its not a fixed thing that you can depend on. FTSE has been leading US lately. Today is an example of FTSE sliding from the open and US trying hard to keep above yesterdays low.
The old saying " when America sneezes Europe catches a cold" ( or whatever the correct phrase is!) certainly applies to the European markets. (obviously) during the pre US market open, the European Indices follow the futures to a small degree as the liquidity is much less than during the main US trading session. When the S&P opens ( cash & Futures) the European markets have priced in the initial net change on the day, but should the US markets move significatly from there, they will almost certainly have an effect on the European markets
If you analyse the major FTSE companies- BPAmoco, Shell, Vodaphone (Airtouch), GlaxoSmithKline, HSBC (Marine Midland) etc you will see significant US businesses. Therefore, there is a logical correlation with the S&P500 and the FTSE100. The FTSE250 with a more UK centric exposure can operate, to a degree, more independently
Mully, BP, Shell, Vod , & GSK and a significant number of other FTSE100 stocks ( and to a much lesser degree a smattering of stocks outside the FTSE100) have ADR's trading in the US- and so, have a higher percentage of US ownership than non - ADR stocks. This would also explain the correlation...