To the OP.

Yes it very well may happen, just don't expect it as if they are not in the mood they don't have to give it to you.
It does not hurt to ask at the check in counter!

what would be more relevant is the load on your flight. If oversold and there is a risk of denied boarding and paying compensation they may look to mitigate that risk by moving passengers to an earlier flight assuming availability. If your flight is not chockers it is very unlikely...unless, as you say, christmas spirit is in the air
I have asked previously (Avios domestic booking...had planned to have escort one passenger from LCY to Waterloo and head back so accounted for that in the timings but plans changed so was able to remain at City...was met with a resounding 'no')

For those who have asked successfully, have you asked at the lounge? At the gate? In the US, it is the gate agents who have the greatest authority to accommodate a request like this, and the ticketing agents and lounge staff generally have very little ability to do so unless specifically permitted by the ticketing rules and absent exigent circumstances.

If you are in the First lounge in T5 then you can ask the customer service desk by the Champagne Bar. I don't think it makes any difference who you ask, and I don't think you will get very far unless there was any extenuating circumstances (such as delays / bad weather) or unless you are prepared to pay for the privilege. If you have checked luggage then you can only ask landside when you check your bags, airside is too late.