Hi Anthony,
Welcome to the forum.
The thickness of the head lining is about 2mm and there is a very small gap behind it in most places of about 3mm. Having said that, it is quite strong enough to fix curtain rails etc. You will need to cut the screws to the exact length and pre drill the holes and self tapping screws will be OK. The bulkhead at the port side of the companion way has a similar arrangement but has a gap of about 8mm. Be careful drilling here because the cables from the log/depth etc run inside. You should be able to mount the VHF under the side deck, but again watch for the cables concealed behind the bottom edge of the head lining.
The keel box has a similar lining over it, which can also be drilled in a similar way but there is less gap behind.
Hope this helps.
Regards
Graham Ebb
235/25 Blue Jazz