He may be referring to Residency. Currently residency for many countries is not just a convinience mailing address. Tax laws (tightened due to terror threats) mean that residency is not such a simple thing anymore and many monetary decisions can be affected by your declaration of residency.
My understanding is that you must do a 90 day report "offline" so to speak, before you can do it online, and that is the only restriction. However, I have heard from one person that Samut Prakan made them do the report in person. But I am sure Tod Daniels is correct about CW.
It is more likely that the Airline will deny boarding in the UK if you do not have a visa or proof of onward or return travel. Heathrow in particular is getting quite tough on this.
I have always done it a minimum of 8 days before, just so I don't have to worry about exactly this question :). However yes, the 12th would be correct.