|Nicey replies: Lots of places although it would seem you need to go anywhere that Tony Blair doesn't.
There are cafes in most London stations but they are tend to deal in charmless Danish pastries and paninnis. Your tea will be subject to whatever nameless catering teabag they shove in your paper cup and how hot their water is. Both of these important factors are usually beyond your knowledege or control.
I find the best places to simply be any every day back street cafe which London is full of. If they do bacon rolls or egg and chips then the tea should be up to scratch. The really good ones have giant teapots and use giant catering tea bags. I was very excited to be presented with one of these giant tea bags when I went to the Fields Cafe in Dalston.
Then of course you should check out definitive back street cafe site eggbaconbeansandchips and its sister site ateaandathink.