How much does the government want to know about us?

So, I'm going to the bank today with a cheque for about £3,200. A cheque, from a UK bank payable to me, not a money order from Nigeria, not cash in an envelope, but a cheque, meaning it's on the radar.

The clerk says to me, "that's a lot of money"

Me: "No it isn't"
Clerk: "Did you sell something to get it"
Me: "No"
Clerk "Then where did you get it"
Me: "The Alliance & Leicester"
Clerk: "No, I mean where did you get the money originally"
Me: "It was an ISA I closed, why?"
Clerk: "The government wants us to ask questions to stop money laundering"

Now forgive me, but if I was ever motivated to launder money, would I really put it into my own bank account, and do they really think a fat stupid bank clerk is gonna stop me? Come on!