WE HAVE all watched horror films and screamed don't open that door...it won't turn out well. This week the Prime Minister seemed to slam the door on a 'no deal' Brexit. But in the spirit of a horror film she did not manage to turn the key on those in her own party and the DUP who insist political principles trump economic realities.

For now Theresa May has seemingly accepted that the split in the Conservative party cannot be papered over and that it is better to go down in history as the leader who opted for country over party. This has echoes of the Corn Laws in the 1840s. Then the party stood up to landowners, repealing laws that taxed European grain to the disadvantage of the majority. These were repealed, the party split and it was out of government for 30 years. That split was inevitable, and so too was this week's showdown with those who have undermined successive Tory leaders, right back to Margaret Thatcher.

Where that all leads time will tell, but in reality most people outside the Westminster bubble are more interested in seeing Brexit sorted out, one way or the other, to end uncertainty. This is not helping farming, and just about every report on market prospects includes a comment about the uncertainty of Brexit.

That applies as much in the EU-27 as here. As an example Tom Tynan, the ex-Irish dairy industry executive adviser to farm commissioner Phil Hogan, recently told a conference things looked reasonably promising for the dairy sector. European production is back under control and global markets are growing. There were however two uncertainties that were key parts of the equation.

The first was the degree to which Brexit would destabilise markets, and no member state is more vulnerable to that than Ireland. His second concern, interestingly, is a growing anti-dairy sentiment amongst younger consumers, linked to concerns about animal welfare and the environment. The industry response is based around claims that it offers sustainable production and processing methods. It believes these are worth a premium. However Tynan poured cold water on that, warning that sustainability was now a baseline to do business rather than something that commands a premium price.

It will be nice to get through Brexit to focus again on issues like this. However, how we focus on them depends on the Brexit outcome. If we are to remain in some form of customs union with the EU, which the Labour party wants, things will be very different on the trade front.

However before we get there Jeremy Corbyn will be tested to see if he can emulate Mrs May by putting country ahead of party and his own far left ambitions. If we are in a customs union with the EU we would trade on the same basis as now in the single market. However – and it is a big however – we will not be there of right, as is the case now, but on the basis of a special treaty for a non EU member state. On that basis attempts, for example by French farmers to block lamb imports, would be easier and we would not have the automatic protection enjoyed by an EU member state.

With a customs union it would be open to question whether and how the UK could make new trade deals beyond the EU-27. Brussels already has deals with a lot of countries that affect agriculture, and it is committed to finalising many more now in negotiation.

Its trump card is the offer of a growing market of 500 million affluent consumers. That is a card the UK cannot play. If, for example, the UK is seeking a trade deal with Japan, which has one of the world's biggest with the EU, the inevitable question would be why would Britain want to do that if it is already in a customs union with the EU?

On that basis, whether you like or loathe a customs union depends on whether you believe the UK can cut better deals than Brussels, despite having only 60 million consumers to offer. One big positive for agriculture would be that a customs union deal, in any form, would block cheap food imports, which remain one of the biggest potential threats of a 'no deal' or even the withdrawal deal Brexit plans. For now, however, the lock on that cellar door is not yet in place.