There are many options, and it is up to you, and any electrician you employ which are used. Wylex do a small size RCBO
it may be these will fit in the existing board? the old one
was taller and would not physically fit in some boxes, at around £32.50 not cheap, but neither super expensive, swapping the existing RCD for an isolator and using RCBO's is likely an option, I say likely as does depend on earthing type etc.
But new board with cheaper make is also an option, some makes of RCBO are much cheaper at down to £14 but a consumer unit is a type tested distribution board and to keep the type tested label you can't use different makes, down to how jobs worth the electrician is.
As you say MK not cheap, but MK do a RCD socket at 10 mA, again an option, but there are loads of ways of doing the basic same thing. Mothers house two RCD's and 6 RCBO's never had one trip, my old house 2 RCD's would get a bout of tripping then years with no trip, this house are RCBO had trips due to water leaks, but no random trips.