The simple answer is it varies, sometimes they react together, sometimes not. Here's a graph showing the performance of the FTSE 100 versus Sterling since the EU Referendum.
In fact you could argue the two perform inversely, ie the FTSE does better when the Pound does badly:
The referendum result was a surprise, both Sterling and the FTSE fell quite sharply initially (stock markets hate uncertainty), with the FTSE then rebounding and performing generally well since. The Pound however hasn't recovered in the timeframe shown above and to this day is still some 10% down on where it was pre-referendum, as I type this.
The main reason for this, is a lot of UK based companies make money overseas, so the pound doesn't influence their profits to a great degree. In fact a weak pound has helped exports so even UK based companies can make more profit as they are able to increase exports as their products are now relatively cheaper abroad.
I expect once Brexit happens, if it does, the pound will fall further (more so the harder the Brexit) and based on what has happened since the refendum the FTSE could perform even better, but usual caveats apply, ie past performance isn't a guarantee of future results!
Over a long period of time the US stock markets tend to outperform the UK so perhaps have a look at those.