Shopping in the West End I often feel the need for tea, sometimes as a light Afternoon Tea, i.e. with cakes, etc. I wouldn't want to do a big hotel tea, which is an occasion (and you have to wear jacket and tie). Department stores such as John Lewis have self service facilities; adequate, but no more than that.
I think the best compromise are the tea rooms in Libertys near Oxford Circus. Libertys has two cafes, an upstairs one a bit more geared up to food, and a downstairs one that's more tea oriented. My family and I went to the ustars one on Saturday - downstairs was full, with a long queue. My wife had Victoria sponge cake with Assam; I had an exquisite fairy cake with a lovely, flavoursome pot of Orange Pekoe, although you can get a full-on multi-tier Afternoon Tea (£18). Salads, quiches and the like are also available. Previously we've been downstairs, and enjoyed the more extensive range of teas there - we've had excellent teas from Swaziland and Nepal. The prices aren't outrageous, the tea is served well, and the staff are efficient and pleasant. This should should be the standard for tea in London.