I had a similar experience, not crypto-related.

My employer paid me a couple of grand in expenses, which I quickly moved to another account. That triggered an account freeze.

I asked the customer service rep for "why" - couldn't tell me. They then asked me for payslips to prove my story. I told them feck off because they could see the monthly entries marked "Salary" from the same source.

I escalated the matter and it took several hours before my account was unfrozen.

Lesson: only keep a minimum amount of money in UK bank accounts.