Firstly, it's not £15bn, it's a bit over
half that - and even that figure ignores the cost of replacing the EU's various functions.
Secondly, this is a tiny figure compared to the total government budget. Even a small decline in the economy would wipe it, whereas even the more generous forecasting suggests we'll lose around 3% of GDP as a result of Brexit so, in fact, we're almost certain to have less money to spend not more.
Finally, even if you did have this money how do you imagine you would use that money to "secure trade deals"? And, given that we will continue to contribute to the EU until we actually leave, if this money was required to "secure trade deals" that would mean that there would be a long gap between leaving the EU and getting those trade deals.