You are unlikely to be able to get credit, and certainly nothing over £500. You could try to make savings from your allowances and possibly try on Ebay.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk
If you can convince your IP of the need for the laptop they might agree a payment break, which will be added on at the end of your IVA, to get funds for another one.
Other than that see if there are any laptop recycling schemes operating in your area to provide school children with laptops for home learning. Local Facebook Coronavirus Support Pages might point you in the right direction.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk