Send a letter, registered post, cancelling the contract. Subject to the contract it is likely the £100 and/or a cancellation fee should be levied against you.
The main concern is whether they have ordered or are manufacturing kitchen units as we speak, with the expectation of you wanting them installed. Although you talk about 8 March... Get that letter out asap
The reality is that the company will be unlikely to pursue a court case for £100, so maybe cancel the cheque too..