No problem to use your phone in UK, just watch out for roaming, could prove very expensive!
How to Call in London
The codes have been changed in London on 2000. The old codes were 0171 and 0181 . they don t exist anymore and have been replaced by a single prefix: 020. The international code to call the UK is 0044.
If you want to contact a London number and you’re in London, you half just to omit the code area (020) and digit only the last 8 cifries . Of course, If u calling from outside UK there is the rule to remove 0, so if you are calling from Italy, u half to digit 0044 followed by the number without 0. For example, to call us at 020 76368344 from Italy just type 0044 2076368344