I use marcrist hole corers size 26mm but pipe plugs that plumbers use is 27mm.As pipes r usually live so can't remove them. Have to file them a little to make tile to over the plug.Never mind on soft tiles but porcelain is hard to do.What do you use to core these holes?
I use my Rotozip to ream holes larger than what the hole cutter can do.. Also you can rock clockwise the drill as you drill the hole.. this is turn will make the cut hole bigger than the core cutter.
The alternative is to go 3mm bigger and get the 30mm socket.
That is part of the best selling bathroom fitters kit that is £49.99 a kit or £7.15 a drill
Kit contains a 30mm to bore over 27mm pipes (Plus a 40mm, 16mm, 6mmx2, 8mmx2)
The kit will drill hole sizes bigger than you might need (see above)
But most items like this fisher fixing kit are concealed by the actual fitting.
In this case the fisher bolt will be concealed by the sink. So the hole size can be generously larger than the 16mm shown.
Ask the plumbers to use compression or soldered end caps.
if they aren't radiator feeds then turn off water at the stop **** or use the isolator valves (these should be installed)
and remove the end caps yourself for tiling.