EmailNumber10 is useful for a short fast response on a contact form, to the government on any breaking statement, speech in Parliament, interview or article. Put the URL in your phone and then you can react instantly. A deluge of emails will quickly show the Government that an idea is a non-starter.
Probably one of the most useful things Remainers can do. Think of it as the email equivalent of a mic drop.
Update: If the server gets overwhelmed with responses, the s drops off https and you will get a security warning. Better software will give explanations hinting at this, rather than just a red alert warning. Just try again later. It may take a few hours.