There are no post offices in the UK that offer special Postmarks, all special Postmarks are applied by the Philatelic Handstamp teams at three Royal Mail Mail Centres as listed in the postmark bulletin (link below).
Bare in mind that the Handstamp Centre’s impose time limits on most postmarks, with the exception of the tourist/philatelic postmarks.
And if reposting to China, you will need to use the appropriate postage rates from the UK,
I.E. £1.70 or £2.55
Too my knowledge RM stamps are not used outside of the UK.
Crown Dependicies and Territories use their own postal system and will have their own handstamp departments, such as the Isle of Man, Jersey, Guernsey, Alderney & Gibraltar for example.
Hopes this helps, I’m sure other UK postcrossers will share their experiences.