Time Trial / Time Trialling / TT

I read somewhere that your head is not very aerodynamic at all. To be honest though I would prefer not to wear a lid whilst tt'ing. The archaic rules in Scotland dictate that this is not on. Am I right to guess that lids are not compulsory for testing in Englandshire?

Aero helmets, as they are used for racing, which do not however meet the ANSI safety requirements, reduce the aero drag by approximately 2% compared to a bald head or a rubber cap over the hair. The Bell Stratos, an ANSI approved helmet, increases the aero drag by approximately 1.3 % over a bald head. Short hair worsens it around 4.6%, long hair around 8.6%. The leather hairnet helmets which can often still be found with racers - although completely insufficient according to ANSI - increases the aero drag by 6.3%. The wide-spread ANSI approved helmet Bell V1 Pro gains around 9.8% compared to a bald head. So far no measurements have been published concerning the influence of beards.
http://www.sheldonbrown.com/rinard/aero/aerodynamics.htm

I'm sure there's newer studies out there and I'm pretty sure they'll all say aero lids are better than a plain old head so long as you keep them level.