Yes the amazon link is the correct biscuit/cookie.
I think shortbread would crumble too much as its so delicate. Tea biscuits are quite crisp. Like a cookie made for dunking.
Do you have a 6 inch spring form? That would work as well as a ring. I don't see why you couldn't use a 6 inch cake pan too. Anything is doable if we set our minds to it.
If I used a cake pan, I think I'd probably make 1 or 2 foil puller outer strips for easier removal. A trick I learned from a friend who bakes cheese cakes in regular cake pans.
Make foil strips, (width your choice) to fit the sides and depth of the pan and over a bit. I put the strips in a criss cross X ) Be sure to make the strips long enought that you'll have a bit of excess foil to grip. That way you can help ease the cheesecake out of the cake pan quite nicely. I don't see why it wouldn't work for the biscuit cake too.
If you don't want to be bothered with the strips just line the whole pan with foil or parchment and be done with it!
ooh, You know what I just though of?!! Animal crackers & arrowroot cookies. Those are available in US stores. They might be a fine sub. Texture is somewhat the same and the thickness is quite the same as social tea.