travel in china, starting in hong kong

good evening

i will be going to hong kong, as a tourist (british passport/ citizen), in december or january. i understand i can stay there for a maximum of 180 days without a visa.
can i apply, get a one in hong kong, to travel onwards into china, if i should decide to do so?
my departure date from the uk is uncertain, due to work commitments, therefore it is difficult to know exactly when i will be going. i hope to stay 2 weeks, three at the most, before returning home.

thank you