I think I’m correct in saying that TCAS II is not an option on a helicopter – variations in predictive rate of climb with fixed-wing aircraft or something like that.
The SKYWATCH HP (TAS) is pretty popular amongst the rotary fraternity and would be worth a closer look.
Several years ago there was a limitation on the number of TCAS I operators within the UK and although restrictions were eased, it may be worth contacting the National IFF/SSR Policy Board before you plumb for a particular type.
http://www.caa.co.uk/docs/7/DAP_SSM_CAA_Doc_714.pdf