Just my 2 cents, 2 pence, whatever:
Having marshalled at Delta Force (Billericay) recently (i.e. Saturday), I can give a few bits of info that I've picked up, certainly as applies to that site (can't say if it's the same elsewhere).
Own markers appear to be allowed, but only if they run on site approved CO2 bottles and are chronoed before the games.
Own goggles are permitted, with clear lenses only.
As far as I can tell, own hoppers can only be used with own markers. With regards own barrel, you might as well forget it (at least for the Juniperwood site). No player will necessarily recieve the same marker for each pair of games. All markers are handed back to marshals at the "gateway" to the safe zone and then just shoved back on the racks in no order - so anyone could get any marker each game.
Like I say, the above info is as it was on Saturday at Delta Force in Billericay, and other sites and different weeks may be different. I suspect not, given that bookings are managed centrally, but you never quite know. Best thing to do is ring and ask.