Wondering if I'll get flamed for this post, but hopefully not.
I am a small online seller and I'm fairly sure that one of my main competitors is not registered with HMRC (but of course I cannot be 100% sure on this). I wonder about reporting them, however they are not a huge seller (probably turn over £1-2k per month) and therefore may even be under the income tax bracket. Would HMRC even be interested and/or investigate.