I had this exact same problem with 123reg, one of our business domain names didn't autorenew, spotted this and instantly renewed it on the same day.
this domain was used for a website and email for our business unit, took 5 days to come back online, 5 days without a website and email.
123reg didn't send us any renewal notice and our card details are in the system as I buy lots of domains with them (over 50 on the same account as this one).
The response from customer services was "we don't have to send notice of expiring domains and we transition your domain even if it is renewed"
So if ANY domain, expired or not reaches its renewal date, you wont be able to make any changes to that domain (nameserver, dns etc) during that 5 day transition period.
We are looking for a new provider for both our dedicated servers and domains.
stay away.