My only memory of a Shipman, at a boat show, was that your first foothold on entry was next to the galley-sink and that this appeared to be a recipe for a broken ankle. There are other boats like this and I would reject them out of hand.
The cabin is pretty roomy, and you'll love the huge V berth, even if the dinette berth is relatively underwhelming. The cockpit is a cracker though, with 2 pedestal seats and a nice U shaped rear bench, the hartop has a huge sunroof, and is good for most weather, other than really hot days, when a fan may be required.
No real downsides either, as build qiality was good, Osmosis not an issue like, for example, Freemans of the era, and the hull is good for river work, but will plane, although the bottoms a bit flat for high speed sea work.
A decent one's right up to your price limit and beyond though.... /forums/images/graemlins/frown.gif
All on your list are satisfactory boats that have stood the test of time. The Sadler 25 is a bit of an odd one out as it is much smaller and racier than the others. One common characteristic is that they were all available when new as kits or home completion so quality of fitout will vary enormously and the passage of 30 years will have resulted in many mods. Some may still have the original engines, Yanmar 8s, Petters, MD1s etc which are mostly past their sell by date. Lots of other money variables also come into play, eg new sails could be 20% of the purchase price of the boat, new engine 35-40% etc. In the end it is the individual boat that ticks the most boxes that gets the vote, even if it is not the first choice design. Personally, I would go for a Sabre as the design meets your requirements the best, but you will be pushed to get one that ticks all the condition boxes for £10k. Did all my early cruising in one and although a bit dull is strong, comfortable and has the headroom. It also looks very "boaty"!
I too concur with the general view that the Seamaster is the best of this lot bearing in mind the requirement for 2/3 crew for 2 week cruises. The Sabre 27 is the only other contender.
Seawolf - limited accomodation, questionable build quality, probably the best sailer of the bunch but consequently a bit lively.
Sadler 25 - good sea boat but limited accomodation.
Cobra - too sedate (even with a fin) although accom. good but questionable build quality again.
Dockrell - too sedate but accom. OK and strongly built.
I know you probably want to contract your list rather than expand it, but take a look at the Van de Stadt designed Offshore 8m - just over 26' and standing headroom (I'm 6' 2"). Just bought mine, in good condition for considerably less than £10k. She is, perhaps, a little top heavy (that's the headroom!), but the hull looks just right. Mine was built in 1971 - I'm not sure when they stopped building them. There is one on boats and outboards - http://www.boatsandoutboards.co.uk/view/SSM286 (I've no connection, just to give you an idea). Separate heads, chart table (albeit over the pilot berth) and small galley.
It might be worth a look at a Cutlass 27. Another Alan Hill design made by MARCON. Less room inside than the Sabre but nicer (not that I'm biased!) to look at and sails really beautifully. Also generally cheaper. You could just about get 6' headroom - depending on how low you have the cabin sole. You can have a wider area and less headroom or a narrow strip along the centreline of the boat and a bit mroe headroom!