You're fortunate that you live in the U.K.; a tiny country with a lot
of gliders. This means you have a lot more choice and it'll be pretty
easy to go look at quite a few gliders.

If you're primary interest is winning club class competitions then you
should look at what glider most commonly wins and buy one of those
(yes, I know that handicapping theoretically means every glider should
be equal but..)

After that you can read Derek Piggott's recommendations and Dick
Johnson's Flight Test Evaluations for some guidance ( they're all on
the SSA website but you need a membership to access them, the Standard
Cirrus B test is available at standardcirrus.org)

Find out which ships you fit in comfortably. As I said above this
should be easy for you as examples of every type you listed will
likely be present at a club not too far away from you. One of the
reasons I didn't buy a Libelle was that I found it too cramped across
the shoulders. A lot of people in my club wouldn't fly the Standard
Jantar we had because they found it too uncomfortable. You never
really know until you try one on for size.

The condition of the glider, particularly the finish is really
important. This applies both to the actual performance (as opposed to
the performance measured when new) and to future maintenance and
resale costs.

As to specific gliders I can speak about the Standard Jantar, ASW-15B
and ASW19/19B from personal experience.

Standard Jantar: The cockpit is long but not too wide (having long
arms is a plus for Jantar pilots). It has a fairly high wing loading
even without water ballast. The landing gear handle can be awkward to
operate as it has to be moved quite far back and you're elbow may have
trouble fitting between you're ribcage and the cockpit side if you're
average height. It stalls and spins quite sharply but recovers quickly
(at least our Standard 1 did and it's the only one I've flown).
Visibility is alright but not great. The large panel and forward
canopy frame restrict things a bit (doesn't apply to the 3 of course).
Airbrakes are more than adequate. Rigging is not too bad. They were
smart enough to provide pins on top of the spars and a lever to fit on
them and pull the wings together just like a Libelle. As far as I'm
concerned EVERY glider that uses a single main pin and cantilevered
pins on the spar ends which fit into bearings in the opposing wing
roots should have this. I curse Grob's engineers every time I have to
deal with one of the club Astir's because they didn't do this and it
makes thing much much harder than they should be.

ASW-15B: (what I currently own and fly) Adequately roomy cockpit for
me (5 ft. 10 and 170lbs.) but not as roomy as the ASW-19 or Cirrus.
Very docile stall spin characteristics, extremely strong airbrakes
(flying at a gross weight of 730 lbs. full airbrakes and a 50 degree
dive doesn't even get me to maneuvering speed) and it can come down
fast using a forward slip. Climbs really well but won't penetrate like
a 19 or Jantar unless it's loaded with water. With the forward hook
and large rudder aerotow is easy. I've never tried the offset C of G
hook. Rigging is easy. The aileron and airbrake connections are easily
accessed and SEEN through the top hatch and the elevator is automatic.
The all flying tail is well designed and doesn't make the glider
twitchy at all. Nice light and responsive ailerons with little
friction. Technical and parts support has been good too. The large
single piece removable canopy is a bit of a pain though.

I live in Canada so a huge factor in my deciding what to buy was the
location of the glider. The fact that the 15 I bought wasn't 3000
miles away or in another country was a major point in it's favor.
There's only about 600 gliders in the whole country and the majority
of them are on the other side of the continent from me. That's not a
problem you're going to have though.

If I were in your place I'd probably go for an ASW-19 or 19B. Most of
the good points of the 15 plus a bigger cockpit, better penetration
and more convenient canopy opening mechanism. The airbrakes aren't as
strong (even with the double blades), the control access hatch is a
bit smaller and the elevator isn't automatic but that's about it for
downsides. It looks beautiful and flies beautiful.