There are many factors for the UK's £100 billion budget deficit, but in my opinion, Brexit is certainly a large contributing factor. Without Brexit, the country would not have experienced such a devaluation of its currency and there would not have been such widespread investor fears and stock market instability.
Official forecasts produced by the Office for Budget Responsibility suggest low tax revenues, slower growth and reduced investment after Britain's vote to leave the EU mean the UK’s budget deficit will soar in the next five years, according to reports in the Financial Times. Instead, the UK is on course to be running a significant deficit in 2019-20 – and things are likely to get worse still. New forecasts are believed to show the gap between the borrowing estimates outlined in last year’s Budget and the actual borrowing now required getting bigger every year.
Across the world, governments "lose" billions of dollars (or pounds) all the time. This is due to the nature of the way that the money comes into existence in the first place (via fiat and opaquely via computer). When you're allowed to print your own money, it has a funny way of disappearing into a black hole. Brexit is simply the scapegoat that the government uses because they know it's a contentious issue that will divide the people and shift the focus away from where the money actually went.
People who don't like Brexit are quick to blame Britain's problems on it. If anything, Brexit is a sign of hope for the people in the United Kingdom. The people in the UK have a stronger economy than continental Europe. They were sick of continental Europe being an economic drain. Brexit isn't to blame.