I'm coming a bit late to this, but here are my thoughts anyway, for what they're worth.
First, The Writing Desk is both a UK seller and offers excellent service and will certainly check your pen before sending it to you. If you want to give the M805 Stresemann another go, then they are a good option.
Second, whilst I have (ahem, n) Pelikans and very much like the M80X model, the value-for-money equation has definitely changed in recent years, as their prices have risen sharply and my perception of their quality control has declined. That's why I no longer buy new versions of anything other than (ocasionally) their 200 series (which are better value and more reliable).
As an alternative, a lovely pen in a different style that writes excellently is the Sailor Pro Gear (large) which costs £216-238 (depending on finish). Personally, I find Sailor's Medium nibs to be close to the platonic ideal for stiff nibs. The ProGear is a little smaller than the 800, but plenty big enough for my largeish hands. Having said all that, I don't particularly care for their broad nibs (by comparison, the Platinum broad nibs are excellent).
In the UK, The Writing Desk and Write Here in Shrewsbury sell them (as does CultPens, I see). I've bought from all of these and find them very good.
Third, I don't know where in the UK you are, but the London pen show is coming up in a few weeks (Sunday 7 October) and I'm pretty sure there's at least one more this year somewhere else in Britain. IF you can go in person, I'd recommend going, and not buying anything before then. Handling the pens in person is really the best way. You 'll have no shortage of inspiration, and you might even find a Pelikan M80X that you really like.
Fourth, you asked about a nibmeister in the UK. I'm not sure the title really fits, but John Sorowka (spelling?) is excellent with nibs. I understand that Omas trusted him with their nib-work in Europe. I _think_ he os oxonian on FPN, but I've seen him at the London pen show.
Fifth, don't assume that you received a return. It's possible that the shop did a dip-test before sending it out, for example, which would explain the scrunched-up plastic cover.
Finally, definitely send the pen back. If you have to eat the cost, then it's a price worth paying for the experience.
You'll have to decide separately whether to buy from the retailer again. I received a faulty pen from a particular seller, I think about a year ago (a Pelikan, as it happens) and they refused to cover the return postage, so I simply won't buy from them any more. In your case, it's not possible from your description to tell whether the pen really was faulty.
PS, a cheeky plug: I'm running a Pay-it-Forward at the moment, with some nice pens available. You might consider entering for one of them. (See the pay it forward board, in the FPN community part of the homepage.)