Any idea what this is before I buy it?

It's a folding bicycle. You can fold it down small(ish) so that it fits inside a car boot or inside a carry bag. At 300 quid in that condition I'd say it's worth it, but I don't know the brand. Folders are usually rather pricey. The question is, do you need it? It's not a good everyday bike, it's a (rather good) stopgap. Do you need to travel by car and then go some distance to where you need to be? Do you commute by train and have to walk the last couple of kilometres or so?

This question has been answered a few years ago. Identify this old folding bike that I got from the tip - an early Dahon?

It's a rebranded Dahon folder and quite unusual. I leave my previous answer below

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The history of Bickerton bikes is quite amusing-- you should look it up. They are quite interesting.

The newer model you have here will probably be a better bike than the ''design classic'' original.

This appears to be an early 1990s-vintage DAHON folding bike, branded and resold by Bickerton. From 1982 through at least to 1987 DAHONs had a brace that ran diagonally up from the top tube hinge to support the head tube about half way up, as seen in this image:

This image displays the bike in Bickerton (instead of DAHON) branding, which is very close to your picture.

From 1990 on DAHON removed that brace, and adopted the pattern of fenders (including the molded-in rear reflector) and chainguard that are in your image:

This image is of a 1991/92 DAHON Classic 3 EP203. As you can see, except for colouring and branding, it is nearly identical to the one in your picture.

(Click images for larger views.)

I have found some photos of Bickerton bikes branded "Californian", but the "Bickerton" company logo on them is different, so I don't know which came first: