I've personally used the Salesforce Document upload mechanism plenty of times, including recently and it has worked for me, but plenty of other people have reported issues in the Document Upload thread. In essence you should be able to get your boarding pass fairly swiftly at LHR and it shouldn't be a matter of concern, despite some of the wording. From what I can make out it's best to use Salesforce once the flight is in Fly, but well before OLCI works. The Salesforce team seem to need 3 to 24 hours, depending on workload. Once they send out their email confirming all is well then OLCI just seems to work, at least as far as I am concerned. But as I say many people have reported outcomes similar to you, but on the day of departure things tend to go smoothly. So check back in a few hours, then just allow a few extra minutes to collect the boarding pass if ncessary.