Having quickly skim-re-read most of the thread, I don't really see any overall negative.
Yes, if the non-tax SB situation is not a factor in anyone's particular case, then yes, there are cheaper ways to go. But for someone wanting to progress from "normal SB" to DMA or DMA-like, while remaining within a SB wrapper, then it seems an obvious thing to at least try for themselves (assuming they have the necessary capital).
It's a pity that Prospreads don't seem to come on here very often these days to respond.
Are they still offering Spot FX, and if so, how did people like it? See:
http://www.trade2win.com/boards/spread-betting/65624-prospreads-com-17.html#post947282
Looking at the product list on the site, are the "Currenex" ones the spot Forex? Still full-size contracts apparently, and even higher margin required.
(EDIT: I see from a previous posting that it is the spot Forex.)