to not understand how the Thames Barrier works

[clam]

Just reading a link on the bbc website here and I don't get how closing the barrier can possibly help the level of floods way back upstream at Teddington.
I daresay they know what they're doing but surely leaving it open would allow more water to flow out to sea, particularly at low tide?

Try thinking of the closed barrier creating a giant resevoir between Teddington and the barrier.

Then when the tide's going out they open the barrier and the water in the resevoir flows out to sea.

There is a series of locks upstream of Teddington that can help control the amount of water flowing into the 'resevoir'.

Well, there aren't particularly fierce storms just there, not like on the coast.

the barrier is a giant upside-down-blind that can be pulled up.
the tide is massive in london, just look at the exposed 'beaches' at low tide and the high walls.