If you are interested in Parker 275 things it may be better to post them on that part of the Forum. As far as I know the 275's were made prior to the RCD coming into effect. Not sure if the 285's received RCD classes. I'll look forward to hearing other input.
The RCD requirements were first introduced in June 1998. Craft sold after then should have a CE mark on their builder's plate, together with an identification number for the craft.
As for crossing Biscay, I think Seal & Parker 27's have been that far afield, and have certainly sailed in waters that are that challenging. I think the capability for a trip like that has much to do with the skipper's preparations and seamanship. The original design and construction of the boat is obviously significant, but the standard of maintenance is probably more critical.
Compliance with RCD requirements is no guarantee of capability - there are lots of fora with discussions of this issue!