Where in UK has the code 0044 777?

00 44 is the UK's international telephone country code +44, shown with a commonly used international access prefix.

A UK number beginning with +44 777 is a mobile.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)