are there any requirements to take ashes to China from the UK

Yes, you need to declare it to China customs and prepare following documents:
(1) Health quarantine declaration form for entry and exit of cadaver and skeletal remains;
(2) Identification certificate of the deceased (e.g. passport, seaman's card, pass, ID card or certificate issued by the diplomatic and consular departments concerned);
(3) A death report issued by an official organ of UK customs, or a certificate of death diagnosis issued by a medical and health department;
(4) Certificate of encoffination;
(5) Proof of corrosion protection;
(6) Identification certificate of the shipper or his agent (e.g. passport or identity card).