Are you factoring in taxes? those are some big numbers. Which are the % brackets on capital gains vs income tax on the UK? How did you acquire your BTC? In the US, I've heard if you hold for 1 year, any income tax that would apply expire and only capital gains apply. However the problem would be explaining to the administration that some guy on a forum paid you to post, that you acquired gains through forks, mining shitcoins and trading them on now dead exchanges years ago, faucets which have been dead for years and you  didn't keep any records... all those things add up into a tax nightmare hard to explain as most likely there's lack of documentation that got lost along the way. It's all licit origin, however, there is a risk they think you are bullshitting. It is a clusterfuck cashing out BTC in any scenario that isn't having purchased BTC in a regulated KYC/AML exchange, at your name with registered ID and keeping all buy and sell records. Other than that, you are playing a lottery in which they may or not think you are a dodgy individual trying to launder money. That's the problem for people that obtain BTC any other ways when they want to cash out. For some odd reason some people think they can simply cash out and "the burden of proof is on state authorities, they must prove im a criminal" as if that was a good idea. There's a lot of people that didn't do anything wrong stuck with BTC's not being able to cash out due fear of authorities, since they can't clearly prove the origin. I wonder how much % of that money is part of the BTC that hasn't moved.