The above chart clearly shows the ongoing switch from inverse relationship between GBP/USD and FTSE100 to a direct one. This is indicated by blue lines drawn on GBP/USD (candles) and FTSE (bars).
The switch states that at least in the short-run ...markets will read further slide in cable as a sign of instability!
Cable strength won't help FTSE
The fact that GBP sell-off is no longer helping FTSE does not mean that a recovery in Cable would result in FTSE rally.
Moreover, recovery in GBP could take the wind out of the FTSE bulls, however, further sell-off in Cable in the short-run would signal instability as stated earlier and hence is unlikely to support FTSE
