The FTSE100 does well when the pound drops against the dollar, as about 70% of revenues generated by FTSE100 companies comes from outside the UK. When the pound drops, their revenues (measured in pounds, at least) go up.

The FTSE250 is down 7% over 5 days.

The FTSE 100 is mainly made up of international companies, so is somewhat insulated from the UK economy. It's still taking a hit though.