The more money-conscious punter in search of a good afternoon tea could do a lot worse than head to the National Cafe at the National Gallery, where you'll pay £5.50 for a cream tea or £14.50 for the full works. Run by Peyton and Byrne, the cafe has a wide-ranging menu that, whilst still a little overpriced in this tired londoner's view, is acceptable given the surroundings and tourist-friendly location. That said, avoid lunchtimes and peak hours.
If you're still looking for a more credit-crunch friendly option, the self service Cafe in the Crypt at St Martins in the Fields does an even cheaper full afternoon tea at £5.25. Not quite the same atmosphere when you have to get it yourself though.
For more information on the National Cafe, visit their website at http://www.thenationalcafe.com.