I had a dog-bone spanner very similar to the Halfords one, it was probably 40 years old, since my father used it when he was a keen cyclist in his youth. You can provide a surprising amount of leverage with them, if you use your palm to push on the end, I used it to tighten and loosen track nuts, and similar, and never had one come loose. Of course, it's possible that the Halfords one is made of cheese and would bend under this load, but I haven't seen the specific spanner.
I'd still be using mine, but I had to do some work on a colleagues bike, after a shop had managed to bugger some things up on it. The nuts were on so stupidly tightly that I broke the spanner trying to get them off. Luckily I had a big adjustable in my bag as well, but if I hadn't taken those wheels off and she had had a fairy visitation, she would never have been able to get the wheel off, I only just managed to do it.