To play for cash UK you must have attained the legal age. This means you will have to produce documents that prove your age. You must have a personal account through which all your payments can be done. In most cases your account must feature your legal name otherwise the payment processing company will not accept any transactions.
Before you join any website that provides games it is important for you to do some research and find out about their reputation. The company you pick should have been operational for more than one year and must have independent customers that can vouch for their quality in service delivery.
When you join any website be sure to read through the terms and conditions. This will let you know about your own obligations and those of the provider. You must also be aware of the rules and regulations that govern each game.