The 685B is also £40 pounds more expensive in the UK ie about 40% more, and has a weight capacity rating of 8kg v 12kg, but to be fair is the same weight and roughly the same height when collapsed.
How is it being fair to say that both tripods are the same weight? Do you have a different set of specifications to show us? The links I posted to both monopods show :
685B:
682B:
As I said, the 682B is about 3/4 pounds or 42% heavier. I did note that it was more expensive, but considering your sloppiness with numbers think it's only fair to also show the prices, since your "£40 pounds more expensive" may be similarly inflated, if you cherry picked the highest and lowest prices that you could find from different sellers. B&H sells the 685B for $149.95 and the 682B for $110.00, both also shown on the linked pages I provided. That's a difference of about $40, making the 685B about 36% more expensive. That's only slightly less than the 40% difference you quoted but it would still be worthwhile seeing links to the actual prices you've used, as I've provided. The supported weights are as we've both stated, 17.6 lb (8kg) for the 685B and 26.4 lb (12kg) for the 682B, but 8kg is far greater than the OP is likely to ever need, and the 685B would easily be able to support Canon's and Nikon's most expensive large lenses when mounted on their largest and heaviest DSLR bodies.
Canon EF 800mm f/5.6L IS USM, $10,900, 9.9 lbs (4.5kg)
Nikon Telephoto AF-S Nikkor 600mm f/4G ED VR, $10,299.95, 11.2 lbs (5.1kg)
If the OP prefers saving a little money that's a perfectly good reason to also prefer the 682B over the 685B, but the latter really is the better monopod, and a quick look at the spec's I linked to as well as the review (which also contains an embedded video review) points out some of the 685B's advantages. There are reasons, after all, why the 682B is cheaper and its extra weight will make its presence known, especially if it's used with one of the lighter cameras.