In order for you to obtain a work permit, you will have to apply for a Tier 2 Visa.
In order to do so, you will need to obtain employment with an employer which has a home office sponsor licence and can provide you with a certificate of sponsorship.
If your visa is granted you will be tied to your employer for the dyration of your visa.
I hope this answers your questions. If so, kindly rate the answer and provide feedback.
If you have any further questions, please do not hesitate to ask.