3 days?
If you're very lucky.
In Norway - see previous writeup in this node - the banks may not be allowed to earn interest while the cheques are being cleared, but they certainly can in the UK.
There is no practical reason as far as I know why cheques couldn't be cleared instantly. They don't need to send the cheques around the country anymore, it's just a throwback to earlier, pre-computerisation times. The reason they do it is because the money-grabbing bastards earn an absolutely enormous amount of money on all the cheques whilst they are waiting to be cleared. At any one point in time there are thousands and thousands of cheques waiting to be cleared - the money has gone from the issuer's account but hasn't arrived at the payee. During this time the banks earn stupidly large amounts of money by taking the interest on this money (which, I believe, they deposit in their accounts with other banks).
Let me just say that again - banks are money grabbing bastards. Especially Barclays, but I won't go into that now.
On the other issue of fraud. As someone has also said, only cheques above a certain value are checked. I used to work in a bank for a short time, and no checking at all is done on cheques below a certain value (the amount escapes me now). I mean the signature on the cheque could be literally anything, only cheques above a certain value were even looked at. And it was quite a high threshold. As Electricsound says - $20,000 in Australia. That must make up less than 1% of all cheques processed.
But then again the CCTV cameras in our branch (which was the central branch in the district for the company) were never on, and were only activated by the cashiers if "something suspicious" was happening. Apparently it was "too expensive" to have them running all the time(!).