No he's not the voice talent for Dennis, he did some voice overs as a child but never this.
Is The Actor, Raymond Pickard The Voice Talent For 'Dennis' In The UK "Dennis The Menace" Tv Cartoon Series?
Well I had a look for you and imdb.com does not mention anything like this on his cv. so you can safely assume that the answer is no, otherwise it is likely it would have been mentioned.