A genuine offer, distinct from an ITT, can be withdrawn up until such time as it is accepted. Once accepted both parties are bound by the terms of the contractual arrangement. As bumsteer (bumsteer?1!?

) has said, as to whether the person you were dealing with within the company has authority is rather irrelevant and if becomes an issue would be a matter of fact. Their authorty as already stated would be either actual or ostensible. I would suggest that if all correspondence has been on headed company paper from the HR department and
has most importantly been signed, it may also have been cc'd to the HR manager as well, a contract will have been entered into which will be binding. Contracts can be on an oral basis but as you can imagine these can prove very difficult to enforce when there is a breach due to a dispute.
However I would suspect that you as the potential employee would be able to breach it more easily than the employer although you would still be in breach. If prior to your start you decided you didn't want to work for them or got another job you would be in breach of contract. They could come after you for any losses they have incurred in recruiting and appointing you but it is unlikely, how many employers would, few I think. I guess it all depends on the nature of the position you are going for. They I feel would just appoint the next best candidate on their waiting list. In contrast, if they breached the contract by saying they didn't actually have a job for you then you could seek a remedy via tribunal

. For a full and proper appraisal of your position I would suggest you take legal advice from an employment lawyer.