Because of the Gulf Stream, if you get the right micro-climate in the UK, you can easily grow moderately hardy tropical plants. Inverewe Gardens in Scotland is on the same latitude as Hudson Bay and they grow palm trees there. The trick is always having sufficient shelter from the wind. At Heligan it's a narrow valley with a stream flowing through it and I'm guessing that provides most of the necessary micro-climate even without the shelter trees which were apparently cut down in during the period the gardens were neglected lost.

Obviously helped that it was one of those glorious early September days we sometimes get in the UK.