For undergraduate studies, individual unis do not have separate deadlines for replying to offers - the deadline for you to respond to offers depends on when the last offers comes through and is set by UCAS. You should have at least 3 weeks (often much longer) to consider your offers, no matter when they come in. So there is no reason or benefit in withdrawing from one of your choices now!
The above is for undergraduate studies; for graduate studies (e.g. PhD applications) then individual unis do have separate deadlines to respond to offers - usually this will be communicated when you are made the offer. Also part-time undergraduate courses that you apply directly to a uni for (rather than through UCAS) likewise will have individual offers. In both cases it's because the applications are not handled through UCAS.
Anecdotally I would say I think Durham is notorious for making undergraduate offers very late in the "season", so I would just recommend waiting it out. If you're a postgraduate applicant though then check the deadlines for what offers you have and make a judgement call.