Adjustable legs are a bad thing. get the right size and keep things simple.
3 legs are better than 2
Welded legs are better than bolt on
Spring tops are useless for touring loads.
Tortec and Axiom are OK brands and their exped rear rack is quite good.
My Blackburn clone is about as good as my real Blackburn EX1 rack.
Adjustable legs are NOT a bad thing per se. They allow one rack to fit more than one bike with no redesign. What they do require, however, is that the rider mount and adjust the rack correctly and completely, tighten the nuts/bolts and then periodically chck those bolts during a trip. This latter bit, checking, is a safety aspect that should be done with any bolt on te bike anyway. A bit of blue Locktite on the bolts and you're good to go for the most part.
Remember that hundreds of people have toured with "adjustable", sub-$50 racks over the last few decades - notably a few round-the-worlders in the 70s and 80s and more during 1976, more hundreds on the TransAm cross-country tour.
Generally, the difference between expensive racks and clones/inexpensive ones, imo, is price or weight limits (carrying capacity and/or weight of the rack itself) or both. I've had one $200 rack and a number of sub-$50 racks. All did the job so long as they weren't overstressed - even the expensive one failed when I overstressed it.
You can "get away with" a lot less "rack strength" if you're only going to ride on paved roads than if you're going to go off-roading.
Loading your panniers should be the same whether it's a cheap rack or not --- keeping the weight low and as close tot he center of the wheel as possible will put the least strain on the rack itself.
FWIW, I really like the Axiom Streamliner DLX. It is a narrow rack which may or may not suit your needs.
Also I have used the Blackburn EX-1 for a Trans America and some other long tours and found it satisfactory. It can often be found on sale for a good price on the Nashbar or Performance web sites. Not sure about UK sources.
More expensive racks will be made of steel or even titanium and will be relatively lightweight (though still heavier than an alloy rack)
Cheaper racks come in 3 flavors:
-alloy racks (a great bang for your buck)
-cheaper steel racks (VERY heavy)
-even cheaper cruiser racks made of sheet metal (terrible in almost every way and usually only fit the bike they came on)
maybe some more too.
As mentioned above, avoid less legs if you're using panniers. Less legs can leave the bag unsecure which could cause it to shift during riding. One has to ask if saving that extra few grams is worth the loss in peace of mind. More material (in general and all other things equal) can usually carry more weight or deal with torsional forces better.
I opted for an axiom journey rack. It's a beefier alloy rack. If this breaks down the road I'll more than likely go with a steel tubus rack. Just wasn't in the budget at the time.
One thing to notice is that each rack you linked to beyond the first one has added width towards the bottom. This helps fully loaded panniers lay flat against the rack, sometimes allows you to position them further back, and keeps soft panniers from bending or twisting around the rack and ending up in the spokes. It may not be essential, but it's nice to have.
Another thing I like is a 2nd, lower horizontal rail that allows you to mount your panniers a little lower than the top of the rack, which frees up more space on the platform of the rack. I use a Topeak rack similar to this one: http://www.amazon.com/Topeak-Super-T...dp/B000H2NIKA/ but that's because I also use Topeak-compatible bags for my commute. There are others with the same feature.
I kind of like my "mouse trap" spring, but then I agree that if I was carrying any serious weight on top of the rack, it would have limited use. It gets more use when I'm riding around town, but even on tour, most of my gear goes in the panniers, so the spring still gets to hold random items like my cable lock, jacket, or a hat, so I don't have to dig for them.
But carrying capacity is your only real sticking point. If it can handle the weight, you can probably make it work. But sometimes the price difference between "good enough" and what "good" is worth it in the long run. My Topeak rack is an upgrade from a cheaper model, but not that much cheaper. If I had done my homework and bought the right rack the first time around, I could have saved a little money.
Some racks are flexier than others. The heavier the load, the more this becomes an important factor. A cheaper rack may be fine for light loads, but will not support a heavy load very well. Look for a rack that is rated to carry substantially more than your max load.
Durability is another factor that will vary with rack quality, particularly if your route involves bad roads or offroad riding. Cheaper racks will probably have weaker welds, thinner tubing and flimsy mounting hardware.
They all seem pretty wide, and have 3 or more legs, and a 2nd rail below the top. I don't know which to choose, but I'm going to go to the shop (evanscycles) and have a look.
Any suggestions on what to look at (other than fit) when I see them?
If you're on 26 inch wheels and are going to stay there, then a rack that sits lower is better. Seems like a "universal" rack might sit higher than necessary an a mountain bike.
Longer racks might let you push your bags back further to avoid kicking them.
I am very fond of my Topeak rack, but primarily because I can easily add/remove my Topeak trunk bag and Topeak basket. Neither of those items go with me on tour, so if my rack was primarily for touring and I was not considering buying the Topeak rack accessories, I would not consider a Topeak rack. Because of the Topeak easy-on/easy-off system, there is a track that runs the length of the rack. It seldom, if ever, gets in the way or causes an issue, but it could dig into a soft bag on the top of the rack, and it serves no purpose if you're not using the compatible Topeak products.