HT, I think I can see your wood through all the trees
Choose any 3 pin UK tranny, with a 7.5V output and rated at least 200mA, you've probably got a garage full of them. If it's got the wrong end on it, just cut the cables and re-splice, remembering polarity. If it's more than 200mA, you're right, it's the maximum current capacity of the output stages, not the power, as we all know that would be measured in Watts. Christ knows what the warning in the ebay ad is all about, I'd read it as "made cheaply, and inappropriately."
Don't buy that cheap crap from ebay, if you must, pop into Maplins, though I've not had much joy with their variable voltage ones in the past.