laminated chipboard, such as an off-cut of kitchen worktop, is good, because it has a wipe-clean surface, accepts screws with pilot holes, and has a flame-retardent surface. if you can space the board off the wall by 15 mm or so it protects it from damp transmission, and bridges uneven brickwork. I actually use the plastic screw-tops from fizzy drink bottles behind the board to put the fixing screws through (used to have chunky plastic spacers from work but these as just as effective).
Don't know how (if) the rules have changed, but in the good ol' days (before the great split), the supply companies used to insist on spacing off the CU if fitted to an external wall (IYKWIM) to prevent damp affecting the install.
20mm plastic conduit sawn to suitable lengths (15 mm min) make good stand off collars for boards. 25mm or even plumbing waste pipes are sometimes better.
A nice flat board to mount the CU (and any isolators, tail blocks - oh and warning notices)
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