About the units:
20p means 20 pence. £1 = 100 pence, so 20 pence = £0.20.

Roaming rates in detail

Service and rate: roaming data £0.20 per MB (£0.20 / MB) Making calls and voicemail £1 per minute (£1.00 / Min) Receiving calls £1 per minute (£1.00 / Min) Sending a text £0.30 each (£0.30 / Text) Receiving texts: free (Free)

About the travel data packs for China:
They can only be bought in the Giffgaff app (each is valid for 30 days):

£12 for 1GB, £24 for 5GB, £36 for 10GB

Troubleshooting (FAQ)

Why was £0.01 deducted after my first top-up?

Quite a few users have seen this, and the charge later shows on the bill as a "roaming fee". It usually comes from a tiny exchange with the network during activation or verification. It happens once and only once; you will not keep being charged, so do not worry.

Where can I see call records and bills?

You can check on the official usage page: Usage Statement.
The detailed call records and bill usually appear on the website about a week after activation or after a charge.