If you follow the above advice and read Tim Hale’s book (or any other good book), you’ll find that long term investing is not best done by trying to pick a stock yourself, but by trying to capture the average market return, which can be done with a broad ETF.
As for Brexit, no one can predict the future. But if you diversify globally (and probably even if you don’t), it is very unlikely that any political event will undermine the 1 thing that all long term investors rely on: the market will go up, on average, on a long time line, at a rate higher than cash savings.