Hi Ross,
Customer-initiated conversion from LRS to ZRS is not available in UK South. What you can do instead is a Support-requested conversion, however, you need a paid support plan to create the support request. You could consider upgrading to Developer support plan ($29/month) and then setting it to not auto-renew. The idea is to only pay for 1 month.
Please click Accept Answer if the above was useful.
Thanks.
-TP