In reply to gilliesp:
I've been to Lundy 3 times and never known anyone have trouble with their luggage, even taking camping stuff as well as climbing gear. As you say, might be more of a problem if the weather's bad and you have to be choppered back. 20kg is loads anyway, you shouldn't have any problems.
WRT hand luggage, best thing is to take your climbing sack with all your climbing gear. It can take a while to get your hold luggage when you get there as they normally unload it, ferry it up the big hill and drop it off wherever you're staying. If you haven't got your gear with you you'll be waiting around for ages until you can go climbing!
P.S. I'd recommend seasickness tablets prior to boarding. It's a rare crossing, even with mild seas, that doesn't see someone vomiting over the side.