UK has already lost Brexit negotiations before it even formally started

The British Empire has been in decline for the last 200 years.
Moreover, the UK government's decision to carry out its final act on Brexit, namely PM May's letter to EU's President Tusk which triggers article 50 (a framework for leaving the EU) is a high stake gamble that could blow-up in the UK's face. Put another way, in the worse case scenario Brexit could leave the UK economically powerless in the EU, the second largest economy in the world with a GDP estimated to be €16.5 trillion (nominal) in 2016, according to the IMF.

A messy breakup would be detrimental for both the EU and the UK. But with the latter being a significantly smaller economy ( UK nominal GDP in 2016 £1,932bn) the EU would easily win an economic showdown (trade war, currency war) with the UK.

Indeed, the UK Treasury is not shooting in the dark, they are fully aware of the economic fallout from Brexit. Last year the treasury (not a europhile think tank) estimated that Brexit would lead to a 7.5% fall in UK GDP and $45 billion loss in tax revenue.
In other words, with Brexit now in full swing, the UK economy could be tinkering on the edge of a deep recession, according to the treasury.

Furthermore, that is not even analysing the direct and indirect impact of the outward flow of capital from the UK, which the EU is likely to be a recipient of.
Japan, a major inward investor in the UK has warned Britain that its exit from the EU could prompt Japanese financial institutions to relocate from London and listed a raft of concerns from Japanese companies about the transition away from the EU.
As recently reported in Reuters, “..a Japanese government task force formed to respond to "Brexit" also warned of a possible outflow of drug research and development investment from Britain, though it said it expected the British government to handle its exit from the bloc smoothly.”
www.reuters.com/article/us-britain-eu-japan-idUSKCN11A0LU

Japan warns Brexit may drive banks, pharma investment to ...

www.reuters.com

Japan has warned Britain that its exit from the European Union could prompt Japanese financial institutions to relocate from London and listed a raft of concerns from ...

But the likelihood of the UK's smooth exit from the EU could be a pipe dream.
The UK has turned its back on its second largest trading partner (EU trade was worth £353.9 billion to the UK in 2015) with no trade deal.