Biggin charged me £75 but I don't remember who that was.
Nobody really caters specifically for the private unpressurised IFR scene - in Europe it is far too small. So, the general procedure for filling at airports is to find a maintenance facility which does refills of bizjet emergency oxygen tanks. This is fairly rare but they do exist at many big ones - you just have to find them, which is potentially difficult since the stuff is all airside and the last thing you can do at big airports is go for a walkabout airside, poking your nose into one hangar after another. You need the right sort of filling adapter and it isn't a 540 nor anything to do with scuba. It's a small threaded fitting - I wish I had the details. A maint shop near where my plane lives does this refilling (so I discovered just after getting myself sorted with the rented BOC bottle) but he only caters for planes with this special fitting, which I gather is standard in the bizjet world.
What Bose means is that the MH kit uses simple bare-pipe fittings which enable a constant flow cannula to be plugged directly into the first stage regulator. You still need a flow adjuster of some sort in the cannula pipe because the 1st stage output is ~ 20-30psi and if you just poke a cannula into that, the gas will be gone in no time. This is what I do - I have two demand regulators, plus some constant flow (Oxysaver) cannulas which serve as either a backup in case the demand reg(s) jammed, or to serve rear seat passengers.