Reporting Capital Loss for Tax Purposes (UK)

Hi,

I recently sold assets at a loss and want to report this via a HMRC self assessment form, to offset any gains in the next few years.

My issue is, the exchange that I purchased these assets on a few years ago no longer exists (Bitforex).

Apart from my CoinMarketCap portfolio (which doesn’t have these trades tracked, nor would I expect this to be hard evidence of my transactions), how else can I prove my losses?

I have related transactions on my Coinbase account but these are just GBP to USDT, as I had to then move the USDT to Bitforex to buy the asset.

Any advice would be much appreciated.

TIA.