In the Zopa mobile app! It's available on iPhone and Android. You can find out your credit score (provided by Equifax) as well as manage an active loan, if you have one. All for free.
You can also check your credit score with several other providers. Some are free, some offer a free trial period, some are paid-for. A web search will show you the lay of the land.
Keep in mind, credit scores in the UK are not standardised. Different providers calculate your score in different ways, and your score may be different for each one. But on the upside - they all follow similar principles.