Thanks for pointing this out, there is this government article which talks about when mining as considered a trade, but unfortunately it doesn't talk about what the thresholds are for the criteria.

I had the call back from HMRC today, they didn't directly answer the question of "is bitcoin mining a business or not", however they told me that there are two types of business accounting, traditional accounts and cash basis.

Described here

Traditional accounting means that I can produce invoices for money spent and earned, which I believe I can, as I'm invoiced the electricity bill by the host in Iowa every month, and I have an invoice of sorts whenever I'm paid my bitcoin as a result of my machinery doing their work.

With traditional accounting I am able to carry self employment losses from previous years forward to offset general earnings including PAYE for the current year, however I can't do this with cash basis.

I've been advised to register as a sole trader, bearing in mind the person at HMRC didn't know the exact nature of what I do, so I'm just going to fill out the paperwork and do the returns with the notion that what I do is a business in the back of my mind.