Brexit already costs the UK massively. The pound dropped from $1.50 per pound in 2015 to $1.25 per pound in 2018. That is almost 20% destruction of wealth.
https://qz.com/1626291/the-british-pound-is-on-a-record-losi...
https://qz.com/1626291/the-british-pound-is-on-a-record-losi...
What a drop in FX rate does result in is a (possibly minor, possibly major) uptick in inflation as imports become more expensive. It also makes the country's exports more competitive since they're cheaper in relative terms.
Some countries actually see a low FX rate as desirable: reduction in the exchange rate is arguably one of the channels that made QE an effective policy.
In terms of GDP its not what it would have been "Brexit had cost the world’s fifth largest economy nearly 2.5 percent of GDP at the end of last year, compared to its growth path prior to the mid-2016 vote on exiting the bloc."
But at least we still grew a little and "UK unemployment falls to 44-year low despite Brexit fears"
https://uk.reuters.com/article/uk-britain-eu-goldmansachs/br... https://www.theguardian.com/business/2019/mar/19/uk-unemploy...
https://tradingeconomics.com/united-kingdom/government-debt-...