Cucumbers have to be straight and must not arch more than 10mm for every 10mm of their length so people can tell how many are in a box.
(The Sun, 4 March 1998, p6)
Cucumbers do not have to be straight. There are grading rules, which were called for by representatives from the industry to enable buyers in one country to know what quality and quantity they would get when purchasing a box, unseen, from another country. Nothing is banned under these rules: they simply help to inform traders of particular specifications. The EU Single Market rules are identical to pre-existing standards set down both by the UN/OECD and the UK.