Dave Reid, they are not common at all in the UK. BA is the only company I know of using them, though I suspect Virgin do too, but that's about it. The charter and locos certainly don't, and that's the bulk of the traffic.
But while a proper dispatcher has legal obligations, the final responsibility and authority always rests with the commander. My point is that if a commander wants to take more fuel than is on the plan, nobody has the legal authority to challenge that. Taking less is also an option where the plan includes tanking, or a further than necessary alternate, or if 5% contingency has been used and the commander is permitted within the company policy to use an enroute alternate (lower fuel may be needed for performance reasons, or because the aircraft has enough fuel to meet reduced planning amounts and refuelling is unavailable - it's not a common practice).
However, it is unwise to treat ground crew unpleasantly or to ignore their concerns. Most of them work hard and even if inexperienced, may have useful information. Yes, gobby and gash "turn round coordinators" exist, and I have seen one or two and given them a mild kick up the backside where needed, but only when they displayed negligence or started ordering cabin crew about, which is way out of their authority.