As part of your application for the student registration certificate ("yellow card") you will need to send evidence to prove your student status and to show that you can support yourself financially during your time in the UK. What do I need to send? (pdf checklist)
From 20 June 2011 all students applying for the yellow card must demonstrate comprehensive health insurance. The UKBA have acknowledged that the European Health Insurance Card (EHIC) is an appropriate way to do this, as long as it was issued in your home country, but they require a letter confirming your temporary status in the UK, eg giving course dates and confirming permanent address in home country, ties back home, family, future employment etc.
You will need to send a valid EHIC issued in your home country, not in the UK, with this type of letter with your CR1 application form. Please make sure you check that your EHIC has not expired.
Please note: We have recently been informed that it is no longer possible to use an EHIC card issued in the UK to demonstrate comprehensive health insurance. You will need to return home to get/renew your EHIC before you can apply for your yellow card (please speak with your national embassy in the UK to confirm what you need to do).