Surely though the US / Israel / UK are fully aware of Iran's potential capabilities?
True Promise I and II will have been studied to the 9th degree. In addition, Israel's partial success in its response to True Promise II will have been studied in the same vein.
There must be a reason (or several reasons) why Israel thinks it could ride out a serious Iranian response. There is something hidden in all this?
I suppose you are right in some respects. Given Iran has said that it will blame the US for any attack by Israel - are the Israelis hoping for US bases to be targeted? But that seems too obvious? Iran has indicated that they do not believe that Israel could carry out strikes without US assistance. Maybe it's just case of this? Iran cannot be fooled by Trump's distancing from Netenyahu.
I would venture that much more was learnt by Iran than Israel and its sponsors from TP I and II, Roch. It seems reasonable to infer from what is known that the True Promise operations were successful intel gathering operations by the Iranians. Israel was made to expose its defences, while Iran didn't really need to display anywhere near their full capabilities. The partial demonstration that Iran did display proved immune to any Israel/US/UK countermeasures. Iran gathered lots of useful intelligence by launching an attack large enough that Israel had to defend(giving away their capabilities) whilst using mostly slow drones and older missiles(which revealed little of Iran's capabilities). They also made sure to give a hint of those still hidden threats by sending enough precise, non defendable missiles directly hitting Israeli bases. The implied further threat is obvious. I'm not sure what useful intel could be gathered by Israel/US from these operations other than the summary above.
As you suggest when stating that "Iran cannot be fooled by Trump's distancing from Netanyahu", I think Iranian leadership see through this pretend "distancing". The main, not really hidden, reason that this attack is supposedly being planned is because of a concept touched upon some time ago on here, Roch. Israel/UK/US are in zugzwang. There are no hidden reasons, capabilities up the sleeve of Empire; they simply have no move that doesn't put them at a "serious disadvantage".
What we are witnessing is panic and desperation, there is no cunning plan.