Question about 593.5nm wavelength classification

Well, I found one that ships to the UK, but their shipping charge is nuts.

Spectroscope Quantitative | eBay

You could either make one with a diffraction grating, have someone here buy the cheap one and send it to you, or look for "spectroscope" on your local ebay and see if anything similar pops up. If you don't mind not having a calibrated scale it is a lot easier to find them.

How well do those work with lasers? Actually how do you use those with a laser? Obviously you can't point it at the scope so do you look at the beam in a dark room or project it onto say a white card and look at that?

Yep, project the beam, usually attenuated via spreading it out, on a neutral colored non-fluorescent surface.

They tend to work pretty well, but the accuracy and precision of the scale isn't enough to determine what wavelength the diode is, if you didn't already know. It is ideal for gas lasers though to check for other emission lines. So for example you'll be able to tell if the 594.1nm Hene is putting out any 543nm or 633nm etc, but you won't be able to tell if the 405nm diode is 405nm or 410nm.

Ah very cool. The use case you mentioned was exactly what I was thinking of picking one up for since getting a real spectrometer anytime soon is not going to happen.