Couple Questions About The Silver Britannia

I just picked up my first 1 oz silver Britannia.
It's a 2014, and listed as a "Horse privy".
It has the image of a horse along the rim.

I am having trouble finding out much about the details
of this coin series. I would like to start collecting
the series (business strikes), and put them in a Dansco
album to go along with my American silver Eagle business
strike set and my uncirculated Mexican silver Libertad
set. Those 2 bullion sets look so good in a Dansco, I
would like to try with the Britannia.

So, what do I need to know about the series (the business
strikes)? What are the hard years, the keys, and anything
special to look for?

When I order pages for my Dansco binder, what mm size holes
will I need for the Britannia? I heard some talk that this
coin has changed diameters mid-series, is this true?

What the heck is a "PRIVY"?
Do all years have a "PRIVY"?
Are there more than one "PRIVY" during a year?
How does that work?

Does this series only have uncirculated and proof, or do
they get all experimental like the ASE, with reverse proofs,
burnished, enhanced, etc?

Thanks so much for helping out a clueless Yank.
Dave