@cicerone imposter,
The state of the current debate on Brexit in the UK is a bit confusing, in my opinion.
The conclusion that Brexit would "boost the UK by £450m a week" was done by those experts by adding together potential savings to the Treasury from eliminating the UK's net contribution to the EU budget, the (hypothetical) increases in exports from post-Brexit trade deals, the supposed benefits of eliminating some EU regulations (e.g. the Data Protection Act).
Wrong, really wrong, however: the report also claims as a potential gain from Brexit increased exports from a free-trade deal with South Korea.
There is already a free-trade deal with South Korea, since 2011, between the EU and South Korea.
So after Brexit, there'll be no potential economic benefit but perhaps/probably (depending on what comes out in the negotiations) a potential cost.