Identify this old folding bike that I got from the tip - an early Dahon?

RoboKaren has done a great job of identifying the bike.

Here's some historic background, a few good pictures of an example in very good shape and folding instructions. If you need more information please be specific.

After Harry’s [Bickerton] retirement in 1987, Mark [Harry's son] attempted to diversify, importing the new Dahon, and marketing it as the Bickerton-Dahon (and later as the Bickerton Californian) - quite a feat as the Dahon was already being sold in the UK at that time. The Dahon was developed in the United States by Dr David Hon, a Hughes Aircraft laser engineer.The bike was solid and simple, but hardly rocket science, and it weighed a massive 13.2kg, but mass-produced in Taiwan, it was cheap, and Mark was able to sell the machines for a lot less than his father’s more elegant folding solution
From The Bickerton Story

Here's what it looked like new - including the carrying bag

From Car and Classic

Here's how to unfold it up

From Bike Forums

It does seem to be an early model Dahon OEM or Dahonesque bicycle -- i.e., made in Dahon's factory or cloned from Dahon blueprints, but then sold under another label.

The way to identify an early Dahon is that their bottom bracket is above the downtube folding hinge. This particular model is missing the steering brace that helped make the early models less rickety.

Early Dahon bikes aren't particularly rare or valuable at all - especially an off-brand like yours. It's not as if you had a 0-series Brompton. With your bike, you can tell from the one-piece crank that it was designed as an inexpensive commuter bicycle - what we tend to call on this forum, a Bicycle Shaped Object. Later Dahons with their oversized aluminium tubing are much better than these early models.

Just remember that just because something is one-of-a-kind or rare doesn't mean that it's valuable.