The 50 pence coin was issued in 1969 and was the first coin only to be used for that denomination, its equivalent was the 10-bob note. The 10p and 5p were issued a year earlier but were used alongside the shilling and florin as having equal value for a while. I think therefore you could say 50p coin was the first true decimal-only coin.
thank you for your replies,and especially to boxtops for explaining how logically the answer was arrived at.I think the question is open to debate,as chronologically its still the 5p & 10p, but since your answer directly answers the question, who am I to argue with you & Google. Lets hope the answers right, and that they are referring to British currency, not to some other country's currency that had some form of decimalisation long before us!