EU referendum, brexit and the aftermath

I dunno, one or more of:

• ftse 100 less exposed to UK market
  
• Brexit priced in
  
• downside risks not clarified yet (ie traders think Brexit won't happen in any meaningful sense)
  
• irrational optimism
  
• warnings were wrong and there is upside from Brexit when we become 'a normal trading nation open for business with the world'
  
• any set of reasons that have nothing to do with Brexit
  
  
  

Sorry for being late.
The main FTSE100 has few companies that have any substantial reliance upon the UK economy.
Holders of US$ have just been able to buy shares in these companies at a substantial discount. Obviously US investors have US$, but so do the smaller Arab oil/gas states, and the biggest bucket of US$ are in the hands of the Chinese. The cynical could see the post-brexit vote slump in the FTSE100 as a subsidy to the Chinese to continue investing in the UK, with George Osborne as the main cheerleader.
The FTSE250, with smaller companies more reliant upon the UK economy has slumped.