My take:
Israel survives very well on it's own, it has a very robust economy. What it can't cope with is the cost of wars - generally wars it does not start but has no choice but to fight.
Beyond that, the cost of daily defence is extremely high. It probably could not sustain the astronomical cost of maintaining it's iron dome. Over the course of a bad week, the anti-missile defence system might cost something like $100m a month just to stop the constant fire at it hitting it's cities.
So without the ability to do that, it would be left with a choice of either letting it's people be killed or a full blown attack. It would probably need to wipe out Palestine for it's survival. Which I am sure you will say it wants to do or is already doing, but if it wanted to do that it already would have. It has had the means, motive and opportunity for decades.
Iran and other unpleasant players would likely smell blood and join in the attacks on Israel more openly, as would Hezbollah, and perhaps other places. There would be a full scale war, likely horrific bloodshed, and like Britain and the US would be involved as they understand how essential it is for global security to prevent Iran's foothold.
It would be a complete disaster.
That's my opinion only, but I'd imagine the US and UK have a similar take otherwise they would not keep supporting Israel to defend itself.