It could be a varety of things. The UK may be tightening their requirements for imported supplements and what they can/can't be, etc. (much like the TGA in Australia and Medsafe in New Zealand); which would indirectly make it potentially a Customs issue. Otherwise it may be that UK retailers/etailers do not want to import from overseas for a while; or that distributors are cutting back on the brands and product lines that they carry. It could be any number of reasons.