The Pounds fell as an immediate result. Since then the economy has been held up by very strong consumer spending which can only last so long in the face of rising prices and increased capital inflows taking advantage of the weaker pound.
The decline is not going to be sudden it'll be a gradual thing as companies move staff out and new investment fails to happen.
I'm guessing that your let's chill and watch comes from a secure position. There are people who 10% increases in prices makes a massive difference and they can't afford to just 'chill and watch.' This is going to have very big consequences.
The IMF said in 2015 that the pound was up to 20% overvalued though. I recall people talking about it being overvalued for many years before that - in fact, the pound being so highly valued (which was great for the financial industry but not that much else) probably led to a lot of the deindustrialisation over the last few decades which is probably where a lot of this really started.
Anyway, the U.K. is a fairly large advanced economy, and as a bystander outside the U.K., US and Europe, I find it pretty far fetched that what you say will occur, and that the U.K. will have problems long term outside the EU. There might be some short term pain but it's probably for the best long term.
I'm struggling to see why being outside of a large trading bloc is better than being inside a large trading bloc. You've said it is better off long term so there must be advantages to being outside.
EDIT
Just to clarify the UK exports much more in services to the EU than it imports from the EU. Services are exactly the type of thing that the EU is different from other trade deals. The UK loses what it has a surplus on and has to pay more for all the goods that it has to import.
The decline is not going to be sudden it'll be a gradual thing as companies move staff out and new investment fails to happen.
I'm guessing that your let's chill and watch comes from a secure position. There are people who 10% increases in prices makes a massive difference and they can't afford to just 'chill and watch.' This is going to have very big consequences.