I might take a graduated ND filter.  I might install a UV filter (probably
only needed to keep
things from hitting the lens though).
I see little use for anything else.
Jim

Cokin P-system, especially since you can get Lee filters to fit the holder.

You might end up with both types of systems.
A graduated ND filter in a round screw in type of filter is almost
useless.  The "break" is at the center of the filter and that may not be
 where you want to have it positioned in your composition.  Since you
mention landscaped, a graduated ND filter will be a tremendous tool for
preventing burnout or underexposure, especially for sunrise/sunset
shots.  The cokin P series holder combined with a better grade than
cokin filter (Hitech, Lee, Singh-Ray) is a good bet.

The Cokin ND filters have the reputation of not being truly neutral in
color.  Just be aware of that.

Be aware that this type of filter is a resin and is more susceptible to
damage than a glass filter.  Carrying them around is a little bulky, I
keep mine in an audio CD wallet.

I found "The Photographer's Guide to Filters" by Lee Frost to be
helpful.  It's at Amazon.  He discusses round and rectangular,
advantages / disadvangates of both.  He has a lot of pictures taken with
Cokin filters, you can view them as an example, but your eyes on your
pictures is what should be the final arbiter.

It's probably too late to get you set up before the Scotland trip.  Have
fun.

JD