Ref. Coinabse in UK:
Transactions in used to be damn quick. Minutes, even.
A few days ago I popped a bit of fiat over to CB and sure enough it arrived in minutes - but when I went to transfer it to Coinbase Pro as I have done before - it was 'on hold'.
They now put it on hold for five days before 'releasing it'. Just so you know; 'in' is not instant anymore, it seems (at least for me). It's a shame as getting money in fast was a big plus. That's cost me more than a few bucks.
I can't say on withdrawal of cash as I don't cash out on Coinbase, though - I have always used Stamp for that.
Ref: Stamp - they did start to ask a LOT of questions in late '17 about where I the coins I had sold were from. Very intrusive and (at that time) they held a fair sum I wanted out until I answered. I was a little rattled as it was a serious figure. Sorted in the end - but I had to jump through hoops that were far more intrusive than ordinary KYC.
I keep a USD account at a bank in the UK so I don't lose out on the USD - EURO - GBP exchange losses. Then use a Forex broker to get back into GBP when I need to. That said, having USD was useful to buy back at Stamp once we bottomed out.
So far, not too many questions at the bank I have the USD a/c at. I didn't actually say upfront it was for BTC, of course. I just said it was 'USD investment income', which was not a lie.