I agree with this bit - astonishing failure to read how it would go down.
Out of interest, do you have an opinion for why the exchange rate changed so much?
Re your question, obviously markets are in part about fundamentals, but more so these days about sentiment. And in that context, I have to confess I am puzzled why the pound fell as it did. Although I expect it will recover.
There seem to be some strange dynamics going on at the moment. For a start, people do not seem to be recognising that inflation is really not our main issue right now. You have two types of inflation - supply side and demand side. Traditionally, inflation is about too much demand for not enough supply, and prices therefore go up. The normal response to address this is higher interest rates, to reduce peoples' available funds, and to encourage them to save their money - both of which depress demand and lower inflation.
But this is different, it's not about excess demand, it's about structural adjustment to supply costs. Costs of supply have gone up, resulting in high inflation figures. Demand has not got up.
Therefore, raising interest rates, is preciseky the wrong thing to do. Our big risk is recession, not inflation. Inflation will naturally return to low single figures once supply costs stabilize - even if stabilizing at a higher level. If energy costs actually decrease then potentially inflation will fall dramatically, possibly even become negative.
The problem is helping support demand, helping people to pay their bills and have enough left over to buy stuff. For that, we need to keep interest rates low, not put them up!. What on earth is the point of Truss giving the average person a four or five hundred pounds in their back pocket to spend, when a 0.5% interest rate rise could take 2x that off people in terms of increased mortgage cost?
I am surprised the BoE doesn't seem to recognise this. But then again it has a very limited brief, to control inflation. And only 1 tool in the box to do it, interest rates. Nevertheless they should not be putting rates up right now.
My guess - to finally answer your question - is that the markets fear the tax cuts will fuel further interest rate rises.