What London area code 709 for?

The telephone area code for London is 020 within the UK, or +44 20 in international format.

If the number is 020 709x xxxx or +44 20 709x xxxx then it is possible to work out the rough area within London if the next digit after 709 is known.

If the number is 070 9xxx xxxx or +44 70 9xxx xxxx then it's a "personal" or "follow me" number that could be terminated anywhere in the world.

(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.)