Offset expenses incurred in P2P lending etc. against income for self-assessment tax?

[Bizman]

I do UK Peer-to-Peer lending and stockmarket trading full-time and they are my exclusive source of income. I don't have a formally set-up business for this.

I'm just doing my self-assessment return and wondering if the costs I incur for P2P lending - they're not much, but things like the cost of buying and repairing a computer etc. - could be treated as expenses and offset against my income for tax purposes?

Or does P2P lending not allow one to claim expenses?

Also, am I technically a 'Sole Trader' or are there other requirements to qualify for that? (I've studied definitions of a Sole Trader in the UK and seem to qualify.)

Cheers.

[Bizman]

To be clear, I only invest my own money.