If the gun checks out and the bbls are good, I think it's a good price. I would want to pay $12,000 for it, but dealers do have to make money.
It looks like that gun has had some work done on it - reblacking, refinish on the wood, etc - and I would just make sure it's all OK. New BBLs will run you around $15,000, so you need to be certain the ones on it are good - in proof, good wall thicknesses, free of bulges & dents, etc. I would also make certain the gun is on the face, the wood is sound, and the whole thing fits you (restocking is $5,000+).
Also, make certain you can take it apart for inspection. You can discuss this with the seller before you buy it. As long as you're using a top smith, the dealer should not have a problem with this. If the dealer does, walk away. And make sure you get everything in writing before you close the deal.
Guns in their original cases with all the tools are not hard to find. But figure on paying another $1500-$2500 for one fitted out that way. A good Purdey oak & leather case without the tools is worth $1200. All the goodies can easily run another $500+.
Here's a decent looking Purdey in a case with the toys:
http://montywhitley.com/antique_arms_fine_sporting_guns/SHOTGUNS/992I think that's a fair price - maybe a bit high. Making a low-ball offer on it may be worthwhile. The case looks English, even though it's not original to the gun.
If you buy in the UK, be sure to factor in the shipping/importing/brokering/handling costs. They can get expensive. Personally, I would rather put that money into the gun. With the dollar as weak as it is now, I think you can do better over here, too. There are plenty of nice British guns in the US.
BTW: if you get the serial # off of a Purdey, you can email it to them for the details on how the gun was made. They do this for free and they turn around the info real fast. H&H may do this, too.
Good luck.
OWD