Manfred
The GBPS webmaster, Maurice Buxton, has provided excellent pages on this as a starter - see
http://www.gbps.org.uk/information/po-numbers/If you want a bit more detail, on the numeral types and much much more on British postmarks in general, then the Stanley Gibbons publication
Collect British Postmarks would be my choice.
If you want to solely consider the numerals in more detail, then I suggest you look at John Parmenter's books on the
Barred Numeral Cancellations.
Have fun with the cancellations!
Winston