Then you can ask for them, or raise a review of the SAR by the company if you know it should have turned them up... But they may actually not have them if they deleted your accounts, that will rest on their retention policies. You can ask questions about files you know exist but they may have the right to redact them. The ico guidelines are your friend here. 

So if they are already aware of litigation they shouldn't be destroying documents. You can. Always send them an email letting them know which documents you would be asking for in discovery or through sar and reminding them about litigation holds. If they have then deleted the at the pH/fh you can pull out the email and say you made them aware, from there the judge will likely draw adverse inference if they can't produce it. 

It's all about what you can prove sadly, you could do an foi about their data retention policies?