If I am offering a interview coaching session on my recruitment website, do I need a disclaimer so there is no mis-understanding that the service offered in no way guarantees a job, and that it is just a value-added service?
It isn't a requirement of English law that you need to have such a disclaimer (and I would be surprised if it was a requirement under any other legal regime).
The true question, I think, is whether the avoidance of liability in these circumstances requires a disclaimer.
As a matter of English law, it would be difficult to put together a case that a coaching services provider was guaranteeing a job, but the degree of difficulty would depend upon the specifics. Because the possibility of such a claim cannot be entirely dismissed, and also to help manage customers' expectations, I do think would be a good idea to incorporate such a disclaimer into your T&Cs of business.