Short term?
-Keep hydrated. This one is key.
-Take an OTC painkiller for the headache.
-Splash your face with really cold water a few times and go for a brisk walk during a break/lunch period (when I was in office my lunches were pretty small/quick to eat to maximize doing-anything-else time, YMMV).
-Depends on your working environment, but if you have the ability to do at least some of your work standing, stand.
-If you're crazy drowsy and just need some means to stay conscious until clock out, a rubber band on the wrist can help. If you start to space out/feel your eyes start to close, give it a snap. Not the greatest thing, but a little pain can release some useful endogenous chemicals.
-If anyone has little candies in the office, ask if you can take some. The sugar in well-timed doses can help a little.
Medium term?
-When you get home, pack up some dried tea/tea bags of acceptable quality, and immediately place it in your 'going to work' kit (bag, car, whatever) so you have a stash in case you forget to make a thermos/whatever of tea the following (and subsequent) mornings.
-Go talk to whoever stocks the tea at work and let them know they've expired. They may not drink tea themselves, and if no one else has brought it up they may not know/haven't checked in a while.
Long term?
-Ween yourself off of caffeine. Slowly start swapping out your tea stash with herbal teas with no caffeine content (some are quite tasty), and reduce the amount of caffeine you're inbibing bit by bit until it's 0.
-Work on adjusting your sleep schedule.