It sounds like you might have an issue specifically with the hot water supply to the tap in your kitchen. Since the hot water is working fine upstairs and the radiators are also functioning properly, it's likely a localized problem. There are a few potential causes to consider. It's possible that there is a blockage or a valve issue in the plumbing leading to the kitchen tap. You may want to check if the hot water valve under the sink is fully open or if there are any obstructions in the pipes. If you're not comfortable troubleshooting it yourself, it's advisable to consult a professional plumber who can identify and resolve the issue more accurately.