Ladycosmos,
In order to answer your question, coming from UK does not mean they were grown there. For example from Ebay they could be imports into the UK grown elsewhere.
The hundred days of sunshine is not quite right either. They are daylight sensitive plants, like many that originate from South America. Which means they grow perfectly well and rampant too, but they will only flower and produce around and after the autumn equinox. Because they are used to flower when the days and nights are of similar length. Our 15-16hr daylight summer days stop them from fruiting unless they are adapted. And it takes several generations to do so. If your seeds came from a UK source and were grown in UK and seed saved in UK, then adaptation will have already started.
They do very well outside apart from the daylight problem. If they start producing at the equinox and we get an early frost (which kills them) there won't be much fruit, if we get a long autumn and a late first frost, then they will produce much better. Obviously if they are in a greenhouse, you can keep them frost free for longer and this is where the greenhouse recommendation comes from.
But as I said before, I always grow them outside because they are rampant. For someone who has a large polytunnel, then growing inside makes sense, if you don't know whether they were from UK grown seeds.
Hope this helps. More to read here:
https://www.allotments4all.co.uk/smf/index.php/topic,79048.0.html