By virtue of evidence accumulated from Google image searches using "posidrive" and "pozidrive" as search criteria, methinks "posidrive" is nothing more than UK terminology for what U.S. handymen refer to as "Phillips' Head". I don't know how things are in Luton, but around here it would be harder to find a store that didn't have a few Phillips' screwdrivers on the shelf than one that did.