Hi Oldtim Hmrc,
There are no Income Tax implications on the receipt of a cash gift unless the cash gift generates interest or dividends. These would then potentially be subject to tax.
Further guidance can be found here.
Tax on savings interest
Tax on dividends
There is no requirement to prove that the money being transferred into your bank account is a gift. You could retain bank statements showing the gifts.
If any of the money transfer to the UK, from your bank account in China, is from income you generated while resident in the UK, then it will be taxable. If it is money from a gift, then it would not be taxable.
You are free to transfer money from your Chinese bank account to a UK bank account and there is no limit.
Your relatives do not need to provide supporting evidence of gifts they have made. They may wish to retain records of the gifts.