Size matters.
And politics of course.
This is nothing to do with American practice. They use the 350 because they are allowed to. They are also allowed to carry part of a firearms team externally if they choose to. CAA would have kittens. However it should be noted that the number of twins being ordered over there is rising. In general the R44 fleet is a small township patrol craft circling a town the size of Coventry in dizzy circles. Not quite UKP ideas of operations.
It seems that in some [most] cases the original spec. for upgrading to the 900/135/109 required a capability of carrying a firearms team. No-one was really too bothered when most of the current fleet were ordered but 9-11 has made someone pay more attention to the problem. Not necessarily leading to again upping the size of the final choice for each of the trio is going upward to 3,000kg anyway.
It is a matter of argument but all of them failed to carry enough bodies for said firearms team. All could carry a section of officers but not enough far enough etc. Two/three in a firearms team is not really worth bothering with on scene. Five or six makes more sense. Currently he only UKP helicopter with that capability is the Devon & Cornwall BK117.
Of the others the 355 was pointless, but the 900 was undoubtedly the best by far and just about covered the base requirement. Unfortunately it had this little spares/support problem that results in unacceptable downtime. Result is that they might decide to look at an alternative type. Not the 109 or the 135 for obvious political reasons [already rejected being one].
The 135 was nearly there but headroom at the back was an issue and of course climbing the stairs [the high skids] presented its own problems in a Health and Safety point of view.
109 similar [or worse] height/volume of cabin problem.
So that leaves the 145 [and the EH101 I guess!] so this is where they are looking and I will not seek to prejudge the issue.
Politics is a major issue with all this. It would not be 'sensible' for an officer directly involved in ordering one type to be so connected to the purchase of an alternative type that was on the original list. So either there has to be a change of hierarchy or a change of airframe - or both.
I do not think McAlpine have anyone on Pprune these days, but I am not sure.