Entering Shenzhen, after arriving in Hong Kong 4 days prior

Hi, I will be flying to HK from UK (no visa required) and then entering Shenzhen 4 days later( with valid Chinese M visa) via train or taxi or ferry (still to be decided).
Am I correct in saying that I can self test with lateral flow test whilst in HK (within 48 hrs of entering China) and then after the test, do the health declaration?

Thanks & Kindest Regards

Steven