Overseas recruiters often require a three-way agreement between the University, the student and themselves, (as the internship provider), which may cover a range of different things depending on the role and the company.
Will UCL sign it for me?
If the internship forms part of your degree through a year abroad programme, the Study Abroad Office can provide more information on how you can get it approved before you start work.
If however it is not part of your degree and is an opportunity you've found or organised yourself, the Study Abroad Office may be able to provide a modified version of the agreement which confirms that you are enrolled as a current student at UCL and give advice on dealing with requests for this type of agreement from recruiters.
How should I approach the employer if they ask for one?
It's a good idea to talk to the employer directly about what exactly they need and what it needs to say. Most larger companies will be used to receiving applications from UK students and will be aware that not all students can get the recruiter's own version of the Convention de Stage document from their University. If you talk to them about it, you may be able to agree on an acceptable alternative.