Oooo-kay... here're a few knives from U.S. dealers that ship internationally. Even with the cost of shipping, the cost will still, (most likely,) be less than ordering from one of the U.K. sites. Your options open up dramatically if you're willing to order from overseas.
A few things to keep in mind:
-If you want to learn anything more about the knives below, Google 'em. There'll probably be a review or two out there already.
-I'd highly recommend dropping the "has to be able to be attached to the leg" requirement. It's completely unnecessary when you have a sheath of any sort of quality, and sticking with it would eliminate some otherwise fantastic knives from the running. I can't emphasize this enough.
-KnifeCenter and Baryonyx both have virtually flawless feedback; you'll be safe ordering from either of them. However, Baryonyx offers a few things that KnifeCenter doesn't. They inspect every blade before it passes through their hands, so the chance of you getting a lemon is lowered drastically; and, they offer a "special grade" option. (Which is explained on their website.) The former is especially helpful, since returns would be a pretty big hassle for you, I'd imagine. But, ultimately, the decision between the two should come down to shipping costs.
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Sharpening a convex edge isn't difficult, and convex edges offer a lot of benefits to the user. But, if you feel strongly that you need a v-edged knife, ignore the Condor recommendations, and don't get the Special Grade option for the Svords. If you do decide to try a convex edge, spending a pound or so on the Special Grade option from Baryonyx would be well worth it.
-If you don't feel comfortable ordering from overseas, the Svord General Outdoors, Condor Rodan and Varan, and Boker TUF are available at Heinnie for under 50 pounds.
-If you're not willing to compromise on the whole "leg attachment" thing, the Schrade SCHF9 will be your best bet. (Unless it turns out that shipping's cheap enough that the Condor Toloza comes in at under 50 pounds.)
Schrade:
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SCHF9- This was actually designed by a member here. Great, tough knife for the price, and it meets all your requirements.
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F10- Not as good as the SCHF9 IMO, but it'll still do the job. Cheap stainless and a blade a bit less than 6", though.
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SCHF3N- Made out of cheap(er) steel, but it'll still work. Easily the best looking of the bunch, IMO. (Because it's a clone of a much more expensive knife.) Buying it is pretty much a victimless crime, but be aware that it's a stolen design. Meets all your requirements.
SOG:
-Forget SOG, there're better options out there.
Boker Plus:
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TUF- This one's probably too small for you, and it comes with a leather sheath, but it's one of the few good, affordable knives available at Heinnie's, so I figure it's worth a mention. Good quality and design.
Condor: Except for the Toloza and Garuda, these come with leather sheaths that you won't be able to rig a leg strap with. Or, it'll at least be more difficult, anyway.
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Varan- Tough as hell, good quality for the price. And ugly as hell, too. Pretty much the definition of a beater blade. Great knife. Also available at KnifeCenter.
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Rodan- Same as the Varan, but in a smaller size that's more capable of delicate work. Also available at KnifeCenter.
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Garuda &
Toloza- These're the highest end knives mentioned so far. They might be too expensive once the shipping is factored in, but I think they're worth a mention anyway. The Garuda is a bit short at 5", but the Toloza sounds like it'd be pretty much perfect for you at 6". The Toloza is also available at KnifeCenter. They're both just as tough as the Rodan and Varan.
Svord: Same as Condor. Leather sheaths.
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Kiwi Trapper- More elegant, than most of the knives mentioned so far. Not sure if it'd really fall into the category of a "beater" blade, but it'd do fine in the role. Virtually indestructible handles, and L6 is tough steel.
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General Outdoors- Similar to the KT. Also available at KnifeCenter.
