A friend has stated that he believes cash is better because (among other reasons) "It is quicker and easier to pay friends or tradespeople with cash."
Right. Easier, sure, after you've executed the dance of 'who has any change for a 20?':
"I owe you £11.50, right? do you have change, I only have 20s?"
"Ah, you only have a fiver?, ok, I'll give you 20, you give me a fiver and you owe me £3.50 next time we meet and you have change."
Or you could just say "Paypal or bank transfer? Ok, I'll send you £11.50. There, done."