A large part of the relative success of slabbing in the US vs the UK seems to me to be that more people across the pond seem to be in a state of semi-paranoia regarding whether a coin is genuine or not. A small minority with deep pockets actively pursue the best score accolade, but I see far more concerns posted about authenticity. Clearly the number of iffy items is likely to be the same there as here, so are we not as concerned as our US cousins, as aware of the problem, or just more laid back as a culture?
GeoffT who is a member here, summed up the difference between cultures when he said that Americans have a greater tendency to believe in absolutes, which accounts for the need to have a coin graded and a number assigned in perpetuity. A sporting analogy would be the need in the US for overtime to produce a winner, whereas the 5 days spent arriving at an indecisive result for a game of cricket would be anathema.
4000 coins per year is not a sustainable business. I think their big problem is that culturally we are somewhat easy going and relaxed about our hobby, and probably crucially, are less inclined to treat our coins as an investment. Therefore, it will always be an uphill struggle to raise the workload to a sustainable level. It would be interesting to know the number of people in this country who submit coins to NGC or PCGS on a regular basis, as I suspect there are not too many. That would give a better idea as to the viability of a TPG in this country, irrespective of whether it is CGS or someone else.