On the day is on the day of travel itself and not before unfortunately, you won't be able to get an upgrade price the night before.

Remember though there might be nothing available and if going from say Economy to World Traveller Plus there may be limited seating options as I once found out from SAN back to LHR. Even though it was an exceptionally good price I ended up in a middle bulkhead seat which was still OK ish.

If an upgrade is available then it's worth asking the agent what actual seats are available, they should be able to see and that gives you more options and therefore I think it's worth looking at the LHR-SFO seatmaps beforehand.

Cabin upgrades

Upgrade offers at the airport are subject to availability on the day of travel and are offered on a first come first served basis.

To check if an upgrade is available on your flight, please speak to a member of British Airways staff at the airport who can advise you of the current availability and price.

Customers who purchase an upgrade can benefit from the full entitlements of the upgraded cabin, including increased baggage allowances.

Terms and Conditions apply.

https://www.britishairways.com/conte...cabin-upgrades