For me, mainly because of convenience and expense. You don't have to pay membership fees, they accept single coin submissions, and it's cheaper. And there is some respect for CGS grading within the UK. I was under the impression PCGS accepted single coin submissions. I have never myself had any dealings with CGS yet everyone seems to complain about the service they provide and the treatment they receive from the staff there so i just wondered why people keep on dealing with them. PCGS accepts single-coin submissions, but adding the shipping both ways, the cost becomes prohibitive. I’m sure there are UK dealers who can submit coins to PCGS on a collector’s behalf? I know St James does it on an ad hoc basis so long as you consign the coin to them for auction. There may be others too.