I have asked support and they have confirmed that they do not deem any crypto earnings as legal and valid source of income apart from income made through mining.
Can you provide proof of that? A screenshot or something? I find that hard to believe, and if any kind of proof other than mining is going to be considered illegal and they're going to block your funds, what they're doing is stealing. I say this hypothetically because I find it hard to believe what you are saying and even more so when it comes to Stake who has been on the forum for a long time.
If it's worth claiming (either fund wise or spite wise) have you checked to see if you can file a tax return and if they'll accept one. From what I've read about proof of earnings for the UK, it looks like the second most document that's used for things like this (the first being letters of proof from a person's employer).
That’s correct. If the amount worth it then he could probably go for notarised documentation duly signed by legal representative. This is also one of the solid proof for such work and it adds up the legality since it is witnessed in front of the legal which is far more strict.
The problem I see here is that they will want to see proof of funding in cryptocurrencies. It won't be enough to submit that you earn a certain amount of fiat money with a regular job if you don't have some way of proving that you have moved it into cryptocurrencies, and the OP says he earns those cryptocurrencies for doing certain tasks, I understand without a contract or invoice.