Eric, 'However the Ukrainians are showing that even helicopters designed for the battlefield are extremely vulnerable' is a fairly sweeping statement. There is far more to helicopter employment than making judgements based upon seeing curated (and sometimes falsified) video clips on MSM/SM without context. I would postulate that Russian TTPs are poor, and they are deficient in both DAS/ASE and Night Vision equipment / training. After all, there is no sane reason why you would elect to air assault an airfield in broad daylight if you could choose to go at night (I've briefed to do one twice, both daylight, both 'at the rush' and I'm probably only still here because they both got cancelled….). They are also showing limited ability to plan/fight collectively in complex mutually supporting packages. Based on what we're seeing, and aware that much of it is false/exaggerated, those are the conclusions I would draw. I would be very cautious writing off all helicopters flown by all nations as 'extremely vulnerable'. In Vietnam, the US lost one helo approximately every 7000 hours - which stacks up very well against some of the Fast Movers (yes, looking at you F105 community…) and, similar to Iraq/Afghan, about 50% were accidents caused by CFIT, loss of control, heavy landings etc rather than enemy action. I've seen some figures worked out from OS info which suggests that the Russians are losing a helicopter every 75-100 sorties, and we've also no data on how many are recoverable having been forced down - which, let's face it, not many FJs are. Fit the right kit, train the crews properly and learn how to integrate with Fast Movers / FIRES and helicopters are far less vulnerable than the Russians are making them seem to be.
Chopper, as Crab notes, 658's 'secret' cabs are photographed and tracked all the time by the spotter community, and the moment one makes a flare to a fast roping approach all hope of remaining anonymous is lost. The replacement aircraft could easily be a Black Hawk as, if NMH opt for it, in itself it would become 'non-signature'. I would, however, suggest that as they are UK-based assets, a handful of leased AW169s would be sensible, or even H145Ms in pseudo police / VIP markings if there was a desire to leverage the advantages of the MFTS aircraft and 'blend in' with the Met Police.