You would need to remove the cloudflare proxy from in front of your main site to enable us to add that hostname to the certificate. Right now we cannot renew the certificate at all, due to your cloudflare configuration, which sends web traffic to cloudflare, not to us directly:
$ host tickets.lso.co.uk
tickets.lso.co.uk has address 188.8.131.52
tickets.lso.co.uk has address 184.108.40.206
tickets.lso.co.uk has IPv6 address 2606:4700:20::681a:45b
tickets.lso.co.uk has IPv6 address 2606:4700:20::681a:55b
This article has more information about the problems and how to fix: [Common Issue] Why isn’t my SSL certificate provisioning automatically with Cloudflare & Netlify? Are there other problems with using Cloudflare in front of Netlify?
Let me know if you can remove than, and then I can get that alias added to the certificate for you.