Renewing HKSAR passport in UK

中國大使館一般都可以幫你續簽你的簽証。
address:
London
31 Portland Place, London W1B 1QD
最近地鐵站：Great Portland Street Station , Regent's park Station , Oxford Street Staion

Manchester
DenisonHouse, 49 Denison Road, Rusholme, Manchester M14 5RX

Edinburgh:
55Corstorphine Road. Edinburgh EH12 5QE

同時需要準備:
特區護照有關申請表，
3張護照照片（照片要求)，
付款表，
相關身份證明材料原件及複印件。

其他資料：
http://big5.fmprc.gov.cn/gate/big5/uk.china-embassy.org/chn/lsyw/hz/t291781.htm

If you want to renew / apply HKSAR passport overseas. First of all, you should go to the website of Immigration Department (www.immd.gov.hk) to download a form ID843 and also its instruction.
Then, you have to complete the form, and prepare all required document, and also the payment (32 pages passport is HK$370 and you have to pay for the postage cost also). You can find those information on ID852.
After preparing all document, you can go to the nearest Embassy or Consulate of China to hand in the form and document. (In your case, it maybe London, Manchester or other English cities) They will send the application back to Hong Kong.
After the new passport is issued, you will be notified to go back to the same Embassy or Consulate of China to pick up the passport.
For further question, you may find it on www.immd.gov.hk