Have a look at the internship section for details of some popular internship/placement schemes to see whether they are still open or closed http://www.e4s.co.uk/docs/internships.htm
Many have closed for this year already, but some are still open. The high quality structured internships run by big companies are usually open for applications from September and close between December and February. Summer internships start June-July and 1 year industrial placements start in September, so it's best to prepare early.
If you want to work in engineering after graduating then doing an engineering placement will look great on your CV, but if you want to try another industry then an internship is a good way of gaining an insight.
A lot of companies have recently started advertising normal temp jobs as "internships" to make them sound more glamorous. These will pop up nearer the summer but are not as good experience as doing a structured internship that offers training and mentoring.
Good luck with your job hunting, and you can create a job alert in your account to be notified by email when new internships are posted on this site.