The FTSE 100 may show strength at the moment but this is because the pound is making new lows. If you look at the S&P 500 the rally near the all-time high is running out of steam, and of course, any piece of good news on Brexit will rally the pound sharply. These two scenarios are highly probable.
The weakness of GBP/USD has been partly driven by the rally in the US dollar. For some reason, the dollar is rallying. Even the weaker than expected US PPI published yesterday failed to stop the dollar rally. The new high means the dollar is trading at levels last seen in July last year. This really is a clear impulse wave, the consolidation of the last few months was the fourth wave, the current rally is the fifth wave. This means the dollar will peak and reverse, and GBP/USD will rally.
Today we have more news that could move GBP/USD. We have UK GDP and industrial production at 9.30 and at 1.30pm we have US CPI. GBP/USD will rally if UK GDP is strong and US CPI is weak. The FTSE is lagging the S&P, I believe the FTSE is leading the way, the UK index is struggling above 7700 for a good reason, the high beta stocks (FAANG stocks) you find in the S&P are not part of the FTSE. These internet stocks have given a boost to the S&P, the rally in the US has been driven by internet companies. The FAANG stocks will lead the decline when they turn down.
Today GBP/USD is down again but the FTSE is not rallying, it would appear the rally ended at 7790. The pattern is not clear, with many overlapping waves. Right now we need to follow the short term trend and a decline is still the high probability scenario.