what to do to study masters in engineering in landon? any entrance exam? and what are the eligibility criteria ?

Hello vikranth!!  Eligibility criteria to study masters degree is, completion of your bachelor studies. You have to search the university and course data base should be in contact with that university before submitting the application to make sure that all the academic and English proficiency requirements. Registration is free of cost and also check for the Tier 4 Visa requirements to apply for studies in UK. You can apply to the university via the university's official website or through  a system called UKPASS.In London all the universities have online application system to do masters course.. Hope this information might have helped you a little bit.
Good luck!!