Whether or not you can just cancel your rental agreement depends on your contract/terms and conditions you have agreed upon.
Rights of termination agreed upon: Depending on the agreement, you have the right to terminate within the stated period.
Rights of termination have not been agreed upon: this may be the case for short rental periods. It depends on the courtesy of your landlord. Theoretically, your landlord has the right to demand the full rent. A cancellation fee is usually agreed upon. If you can not reach an agreement with your landlord, please contact firstname.lastname@example.org or +44 208068 50 07 and we will try to find a solution.