Double Check Valves in a wet room are a sliiiiightly grey area to some, but my opinion is that if there's a grey area then you should err on the side of caution. DCVs are used to prevent potentially mucky, bacteria-filled, water being drawn back into the mains through the shower hose in the event of a mains failure. Granted, this is only really likely to happen if the drain's blocked and the shower head is dangling in the water on the floor when the mains fails, which is a highly unlikely situation, but regs are regs and we must all be paranoid about these things!! It's especially unlikely in wetrooms as the water would normally end up flowing out of the doorway before it bothered the shower hose, hence the slightly grey area, but better to spend the extra £5 and fit DCVs than not IMHO
Full bore isolators tend to be physically larger, City do full-bore ballofix valves but I'd tend to go for quarter-turn lever valves, which are almost all full-bore and also need no tools to operate.