Just in time to meet her March deadline, British Prime Minister Theresa May has pulled the trigger.

Brexit has started with Britain giving formal notice to the European Union (EU) that it wishes to leave. Now it has two years to negotiate a severance on the best terms it can get.

If the complications that arose even before she pulled the trigger are any indication, Britain's negotiators are in for a torrid time at home as well as in Brussels.

First, the House of Lords wanted the rights of European citizens in Britain to be protected in the Government's Brexit legislation, then Scotland's leaders demanded another independence referendum at some point in the process.

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For all that, the decision Britons made nine months ago looks irreversible.

Of the twin electoral shocks delivered last year, Brexit looks more permanent than the effect of Donald Trump. His idiosyncratic rule will last four years or eight at worst.

Brexit is forever. In the broad sweep of history it will be the past 45 years of membership in the EU that looks out of character for a country that has always kept its distance from continental Europe, emotionally as well as physically.

Yet New Zealand, along with others of the "old Commonwealth", were resigned 45 years ago to the likelihood Britain would be in Europe for good.

Were they alive today, those who worried with good reason about the implications for our dependent economies would scarcely believe the notice Britain has now given.

But they would have no regrets for the changes Britain's original decision made to New Zealand.

It was a defining moment in our history, and not just in economic terms.

The loss of a guaranteed metropolitan market for our farm products forced us to spread our trading wings, find new markets in Asia, Russia and the Middle East and set about trying to broaden our range of export products.

In doing so we became truly independent, no longer a colonial economy and more inclined to shake off the emotional and cultural vestiges of the colonial relationship.

After 45 years it sounded strange to New Zealand ears when Britain's "leave" campaigners extolled the Commonwealth as an alternative focus for Britain's trade.

The Commonwealth is a fine association of diverse nations that mostly value their shared colonial heritage. But it would be a surprise if it is reinvigorated as a consequence of Brexit.

Britain is more likely to follow the trade routes New Zealand and Australia took, to China and other large or developing economies. Britain now has to establish connections with them independently of the EU.

Both Europe and Britain remain important markets for New Zealand, which will need to deal with them separately when Brexit is completed.

Access to the EU may be more difficult in Britain's absence but some encouraging sentiments were shared when Prime Minister Bill English visited Brussels a few months ago.

New Zealand has no more than two years to ensure its trade with Europe will not suffer when Britain departs.

And nor should we take the British market for granted again. We are all independent traders now.