https://nationalcareersservice.direct.gov.uk/ – a huge amount of information on finding jobs and what you need to do to get one (CV, interview skills, skill mapping, etc.)
https://www.gov.uk/browse/working/finding-job – Government advice on anything to do with work (particularly your rights.
https://www.theguardian.com/money/2010/may/08/finding-a-new-job – a complete guide to the stages you should go through when finding a new job.
https://www.prospects.ac.uk/ – lots of information on planning a career, finding a job and what you need to do to get it (CV, interview skills, skill mapping, etc.)
http://youthoria.org/home/work/ – run by Cambridgeshire County Council, this website also offers job opportunities and general advice and support to teenagers.
Subject specific advice
http://www.computerscience.org/careers/ (Computer Science)
https://www.ice.org.uk/careers-and-professional-development (Engineering (civil))
http://www.theiet.org/apprentices/area-engineering/electrical-engineering.cfm (Engineering (electrical))
http://www.imeche.org/careers-education (Engineering (mechanical))
Opportunities for summer placements
http://www.ncsyes.co.uk/ – National Citizen Service, Government funded summer program for 15-17 year olds designed to boost employability skills.
http://www.nuffieldfoundation.org/nuffield-research-placements – Provide bursaries to allow 4-6 week summer placements working alongside scientists. More likely to support those from low income families without a history of going to university – Application date – November for the following summer.