One obvious consequence of this is that if you don't provide a witness statement, you can't provide evidence at trial. This is true even if you're directly involved in the case. So, if you intend to offer evidence, it's vital to write a witness statement (which is purely a description of what you know that's relevant to the case), attach a statement of truth, and send it to everyone involved by the date provided at the Case Management Conference

The usual approach is for both sides to hand over their witness statements simultaneously, in order to avoid the situation where someone is able to read someone else's witness statement and modify their own. These days this seems to be "We will provide you with a large encrypted file beforehand so you have plenty of time to download it, and then we will arrange a time to swap passwords", which does simplify things a little.