I have never seen anything in the NOTAMS warning about the density of helicopters in the surrounding airspace
Well, that doesn't matter because your thorough pre-flight planning also includes current mauve AICs, doesn't it?
AIC M032/2010, para 1.1 - The 2010 Santander Formula One British Grand Prix international racecar meeting will take place at Silverstone on 10-11 July 2010. A large number of fixed wing aircraft operating from Turweston Aerodrome, and helicopters engaged in private and public transport of spectators to and from Silverstone are expected.