00 isn't part of the code, it is a commonly used international access prefix.
Telephone country code +44 is the UK, plus the Channel Islands and the Isle of Man.
+44 11 is an incomplete area code; you need the next digit:
- +44 113 = Leeds
- +44 114 = Sheffield
- +44 115 = Nottingham
- +44 116 = Leicester
- +44 117 = Bristol
- +44 118 = Reading
(The plus signmeans "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.)
