I don't think I could afford to fly if I took up golf with any level of commitment. Its fortunate for me that I hate the game with a passion.
Still, looking at the numbers on Bose-X's excellent website, it suggests there are around 100-200 UK based PPL/IRs(admittedly, more than I thought). This is against a background of 20,000 or so PPL holders, so about 0.5 to 1% of PPLs have IRs. (Numbers are from a quick google, your knowledge/sources may be superior) I wasn't able to find any indication regarding how many of these flew exclusively SEP.
I think this lends weight to my supposition that the number of PPL/IRs flying SEP are close to zero. I don't dispute that they exist, just that they are very uncommon. As for the rating greatly increasing one's skills, it surely must.