Doulton produced all kinds of fired ceramics including tiles, crockery, fireplace surrounds and chimney pots (I have an unglazed one in my garden dated xxx). They also designed and made tiles in all shapes and sizes for room interiors and entrance halls and produced the commemorative plaques in Postman's Park.
Many of London's pubs and buildings were decorated and protected mostly at street level with Doulton's distinctive tiles and many of them still remain.
Hand-firing is (and was) a time-consuming and expensive process and so these days we mass produce by machine but the end product is nowhere near as good. For example, next time you are on the Piccadilly Line compare and contrast the modern replacements, such as at Russell Square; the new ones are flat and just haven't got the depth of colour and lovely crackle glaze that the old ones had.
Doulton didn't appear to have a definitive logo stamp or company signature. As you can see from the pics below, the marks* vary a lot even within the same building. Note especially the middle row of photos which are all to be found in the portico of Lloyds, Fleet Street. I have thus far only spotted two stamps that are alike, shown bottom right, and these can be found in Covent Garden and Fulham.
Another thing to note is that even though Doulton achieved 'royal' status in 1901, none of these marks echo that fact, even though most of the tiles were affixed in decades later in the 1920s and 30s.
Still hunting for more... do let me know if you spot any.
Lambeth x2, Greenwich, Waterloo
Charterhouse, Fulham, Covent Garden