Theres no requirement at Sywell for high viz (although they apear to be on the list). The only time I've seen them them worn (apart for by visitors) is by officals directing at very busy fly-ins with lots of taxying traffic.
This is at the nub of the matter. It comes down to risk assesment, something all pilots are trained to do. Its also known as common sense. When theres a large amount of traffic, both ac and pax/pilots, in a close space it is a sensible precaution to don a high viz. The rest of the time it an OOT requirement. If pilots cannot be trusted to 'assess the risk' appropriately you might as cut to the chase and ground us, rap us up in cotton and lock the whole populus up for our own protection.
end of rant