Six botanical photographs, brought together as a collection and presented as A4 fine-art prints in a portfolio box.

Then a beautiful photograph of the open box with the prints, followed by a few large images showing the six works, a little about the idea behind the selection, and finally the specification and purchase button.

Six A5 fine-art botanical photographs
Archival pigment inks
315gsm archival matt fine-art paper
Individually signed
Presented in a portfolio box
Includes a numbered contents card
£125

Six A4 fine-art botanical photographs
Archival pigment inks
315gsm archival matt fine-art paper
Individually signed
Presented in a portfolio box
Includes a numbered contents card
£210

Then ADD TO CART.

I'd also include one very practical sentence that I think could help sell it:

Keep them together as a collection, frame them as a group, or change the works on your walls whenever the mood takes you.

Where I'd put it on the site

I wouldn't add another major item to your main navigation yet.

Instead, introduce the portfolio on the homepage with one striking image and a small DISCOVER THE COLLECTOR'S PORTFOLIO link. It can also appear on the Prints page, perhaps before or after the regular print collection.

That gives you a nice hierarchy:

Prints → browse individual works
Cards → affordable/sendable
Collector's Portfolio → the special, premium piece

And I think that's potentially quite important for the brand. A £200-ish beautifully photographed portfolio box sitting on the site changes the context in which someone sees a £25–£65 individual print. Suddenly those individual prints don't look expensive at all — if anything, they start to look very accessible.

I would start with only one portfolio, though. Make it your definitive selection of six rather than immediately producing Fern, Floral, Leaves etc. If people respond to it, you can develop further portfolios later.

And I'd call it simply The Collector's Portfolio. No Botanica, Naturalia, Herbarium Collection or extra naming gymnastics required. The simplicity suits where the site has ended up.