Common ragweed is not a UK native species
Ambrosia artemisiifolia is native to the Americas
I found it growing in the cracks on our patio. We enjoy feeding the birds so I expect it came in with some bird food. It’s also known as Annual or Low ragweed.
The plant was about 40cm tall when I spotted it in September. We had been away for a fortnight so I guess it had a chance to get established before we got back! It had divided foliage with silvery hairs hence the name artemisiifolia. The flower spikes were about 10 to 15cm long and were holding cream coloured insignificant flowers.