The distance selling regulations were replaced by the The Consumer Contracts (Information, Cancellation and Additional Charges) Regulations 2013.Whoosh...
Send them an email, don't reference any previous correspondence. Under distance selling regulations you would like to return item - where do they want it sending? Yours faithfully... Done.
The whole powered on thing is a red herring imo. You'd have an opportunity to see a device powered on in a physical premises, and DSR is intended to allow online/remote customers the same opportunity to inspect goods as in a store. What's to say your visual disability doesn't make this device unsuitable, having inspected the good as you would in a physical store? Hookum. Just keep schtum and return it as I said.
In a physical store the OP may have had the opportunity to see a display item powered on but he wouldn't have the opportumity to break open the sealed packaging of a new item and try it out.