The garden is located in Cymru Gogledd (or North Wales, if you prefer). The plant itself is a little under 2' / 24" / 60cm tall, the leaves (which are quite large) are soft and hairy, and the stem is deeply grooved, very fibrous, and hairless. When the leaves (which are quite large) are crushed they give off a slightly minty smell, and the plant also likes to have a lot of water.
Looks like it could be Perilla. Does it have a strong taste, like anise and cinnamon mixed together?
Here's an out of the ballpark suggestion: Helianthus tuberosus (Jerusalem Artichoke). Usually multistemmed if from a tuber but if from seed you may have a single stem. Test: are the leaf petioles winged? If so, possibly Helianthus. As plant gets taller as it will, leaves change from opposite to alternate. Can get very tall. It might explain why the stem is so vigorous and beefy.
Edit: Natasha already mentioned my first suggestion here, but I didn't see it before I answered.
It's possibly shiso (Perilla frutescens), which is in the mint family. It looks like that, anyway. It grows wild in the eastern United States, but I'm not sure if it's in Wales (although I imagine it could grow nicely and reseed there)—sorry for the potentially offensive typo earlier.
Shiso has a number of uses (edible leaves, food dye for red kinds, etc.), and you can buy it at vegetable seed stores. The link gives an example with very large leaves, which looks similar to yours.
It also looks a lot like stinging nettle (but much more like shiso). I've heard a rumor that stinging nettle likes much water. Stinging nettle isn't in the mint family.
It's difficult to say what it is for sure, but my opinion is that it's likely shiso, or another Perilla species, considering the deeply grooved stems, the resemblance, the minty smell, and the huge leaves.