When traveling to Britain, is it better to get a mobile phone card locally or to use the portable wifi bought in China?

First of all, you should refer to your travel days.

If you need to call China frequently or surf the Internet by mobile phone in the UK, you can consider choosing SIM cards provided by operators such as Giffgaff, Vectone and Lebara. After these SIM cards are recharged to a certain amount or added to the monthly subscription package, they provide unlimited monthly subscription packages.

It is still appropriate to apply for a local mobile phone card.