Generally employers look for following qualifications in your field:
-- you may be able to become a CAD technician by starting as a trainee with an engineering or construction firm.
-- you may need some GCSEs (A-C), such as maths, science, engineering, design and technology, etc - but enthusiasm for the industry is often more important than qualifications.
-- also 1-2 years experience is more important than qualifications
You may take college courses that covers computer design skills, for instance:
--BCS Certificate in 2D Computer Aided Design (ECDL CAD) Level 2
-- BTEC Certificates and Diplomas in Engineering levels 2 and 3
-- BTEC National Certificates and Diplomas in Mechanical, Manufacturing or Civil Engineering Level 3
Further training and development that may be helpful:
-- training on the job in specific CAD software relevant to your industry: PDS (Plant Design Systems), etc
-- work-based qualifications which cover CAD:
NVQ Performing Engineering Operations levels 1 and 2
NVQ Engineering Technical Support Level 3
EAL Advanced Diploma in Engineering and Technology Level 3.
You may start to work towards general degree in engineering, construction or civil engineering depending on your field.
You could register with the Engineering Council to gain EngTech status, which could help your professional development.