Written by: George Prior Sterling will rally and the economy will have a growth spurt should UK Prime Minister Theresa May’s Brexit deal pass Friday – but this could be reversed if Boris Johnson becomes PM.This is the warning from Nigel Green, founder and CEO of deVere Group, one of the world’s largest independent financial advisory organizations, on the day the UK was originally meant to have officially left the EU.His comments come ahead of UK politicians voting on Friday on the withdrawal agreement - the legally binding document that sets out the terms of the UK's departure from the EU. In a bid to end the impasse, the government has split the original deal into two parts: the withdrawal agreement and the political declaration.Nigel Green notes: “MPs will vote today on whether to accept the Prime Minister’s version of Brexit, face crashing out without a deal on 12 April, a much longer extension, or a general election. No-one – really no-one – wants this uncertainty to continue.“Since she vowed to quit, if has she gathered some more support in parliament, there is seemingly a gradual shift towards Mrs May’s plan as it will allow the UK to leave in, supposedly, a more orderly manner.”He continues: “Should the PM’s deal pass on Friday, sterling will rally and the economy will have a growth spurt as pent-up household spending and investment kicks in.“Domestic-focused small and mid-cap stocks would outperform large caps in a stock market rally.”Related: The New Ultimate Luxury Item for HNW Individuals He goes on to say: “However, the economic future of the UK now really depends on Theresa May’s successor.“Boris Johnson, who is now also widely expected to be backed by arch Brexiteer Jacob Rees-Mogg amongst others in the leadership contest, is seen as the likely winner. He would push for a much harder Brexit. This can be expected to create more uncertainty, spook markets, and negatively impact sterling and UK financial assets.”The deVere CEO concludes: “Whatever happens in the UK parliament with today’s vote, or on Monday’s, many question marks remain. We don’t know who the successor, the one who will shape the UK’s future relationship with the EU and global trading partners, will be, and what their approach will look like.“This is why to mitigate risks and to take advantage of the potential opportunities the volatility is creating, investors must ensure their portfolios are properly diversified.”