Well, hang on.
Firstly, Brexit hasn't happened yet, and nobody knows the plan. Every time the PM asserts something about where we're headed plan-wise, the markets tank. Once Article 50 is invoked, you'll see GBP slide and FTSE will get... interesting. Because a lot of the FTSE is made up companies with a lot of different EU dependencies, it's hard to call where it's going to go. But it's not likely to be good for most investors.
As for Trump and the stock markets? Well, suggesting an "immediate crash", that was silly.
Right now though, the US is in the same place as the UK is with Brexit: nobody is quite sure what's about to happen. He's made a lot of promises that he might not actually be able to keep. If he does, it's likely to cause some serious mayhem in the markets. Not least: he's specifically said he wants to devalue the dollar.
Just let that sink in for a minute: the sitting President of the United States of America got into the Oval Office by promising to lessen the dollar's buying power and value.
The hope then is that this will make US shares more favourable to overseas investors, imports more expensive promoting internal trade (and therefore jobs), and therefore US business more worth overall.
Will it work? Who the hell knows? None of us. And that's why it's too early to call those predictions of decline, wrong.