Documenting bitcoin cash buys I've been trading BTC in the UK via bank transfer for a while now and may begin doing cash purchases soon. I've realised that it is critical I document the cash withdrawal as an expense so that I get taxed only on the profit and not the entire turnover of the bitcoin sale. Usually this is done via the means of keeping receipts - so I guess I could have some sort of receipt that documents the exchange but I'm not sure where to begin.

What kind of information do I put on it? 
What is the bare minimum information I need on the seller?
Considering it is me issuing this document it's not exactly an invoice or a receipt, is there a word for it?
Begin by asking your local tax authority.
They would be the ones able to give you a proper definitive answer.