Certainly all Prem's and some Golds are preassigned whatever the flight. Therefore if a Prem wanted 1A, SS; the despatcher or Station Manager would already have preassigned it.
Most of the time a Prem or high Gold would also have had the adjoiining seat 'blocked-off'.
It is really best for Prems and high Golds not to use OLCI. The problem you have is that you cannot tell that the seat next to you is blocked-off, only that it is not available. Often on a relatively quiet flight, I have made the assumption that someone has used OLCI to bag 1A (on an A310 my favoutrite being 1C) and have moved myself from 1C, only to get to the airport and ask why I have moved from 1C when 1A was blocked-off for me!
By the way, I had a very interesting chat with a BA Manager on a flight from DEL-LHR the other day. He said he was responsible for BA's client facing web-service including OLCI. He said that around 35% of OLCI attempts fail and that BA have four full-time employees in India just correcting data associated with OLCI and are in the process of hiring more. With Telephone check-in no longer available to the masses, BA seem very concerned about the high failure rate of OLCI attempts.
He asked for comments about OLCI and the BA web-site in general, and seemed genuinely interested in my comments and suggestions.
I am ready to take the credit for a small list of enhancements if they ever happen!