Jimi has a point…and some areas will increase chlorination during a certain time of the year, every year. In the United States you can look up the contaminants in your tap water on a government website. I'm sure there is something similar in the UK.

Since this has been happening in the winter, it may be that increased heating is drying out your air. If the problem is particularly bad in the morning you could try a humidifier (many humidifiers will have a special cup for pouring in cough suppressant that could soothe your throat).

I assume you've ruled out bacterial infection (like strep throat)?