Helmet For Convertible

[Wollemi]

I have a Boxster S and have done a couple of track days with PCGB which were great and now I want to do more. So far I've hired a helmet but maybe now now I should be thinking of buying my own.

What are the rules about open face helmets in soft top cars (I always prefer to drive with the roof down anyway if that makes a difference)

At the two days I've done it was fine to wear an open face in my car but I've heard that some organisers / circuits insist on full face helmets.

Any advice as to what to buy?