Where can I get a Starbound Observers Chair or near equivalent in the
UK, preferably by mail order?

I can't find a store specifically located in the UK, but I have found
it listed in the US.  A simple email to them will tell you if they are
willing to ship overseas but other than the cost of shipping I can't
imagine it would be a problem.

Here's the link to the item you're looking for:

http://www.optcorp.com/cart/ProductList.asp?SearchFor=starbound&image.x=13&image.y=4

Since you mention "near equivalent," you may want to consider the
CATSPERCH Observing Chair, which I have found to be more versatile and
more comfortable than the Starbound, yet lighter in weight.

http://home.earthlink.net/~flyj/cperch1.html

This chair can be ordered fully-assembled or in kit form. Although it
is built in the USA, the home page for the CATSPERCH includes this
statement: "WE SHIP WORLDWIDE! Shipping IS EXTRA! Please provide me
with your shipping city and postal code to determine the total cost."

Thanks for the suggestions. It's strange that no-one appears to be
importing this into the UK - the only observing chair that UK dealers
seem to stock is the quite different TeleVue Air Chair.