Your UK ATPL will not have expired yet.
Unless it contains a valid Class or Type Rating and a valid JAA Medical Certificate, you will be unable to use your licence privileges.
You may also need to renew/revalidate your Class 1 medical.
You should not need to complete any MCC course if you hold a UK (not JAR-FCL) ATPL.
You do NOT need to convert your UK ATPL into a UK-issued JAR-FCL ATPL.
You'll need to renew both your IR and (presumably) MEP Class Rating on something like a Seneca. Also consider renewing your FI rating as it may make you more marketable when you leave. You might be able to use ELCs for this - PM me if you want to do this and I'll aim you at someone suitable!