The 225, although I personally like the aircraft, is not an option for UK SAR mountain flying. It appears to be optimised for those long heavy crew change flights and it's unlikely it would have the required lift and hover ability unless AH did some clever mods.
As for the AW189, you have to remember that it is currently out there doing the job well in two British territories on contracts with two different UK departments of state, so it scores a lot of points for that. Once you run the numbers for mountain and maritime in hostile environment SAR, the number of options get less and less.