Do you think that the police would keep a database of all the people that walked in off the street stating that they intend to disappear but not to start looking for them!
Unless starting to work illegally, you would need to obtain a new NI number, unless the same one is used after changing names via deed poll. Would your State Pension entitlement have to start afresh if a new NI number is issued? Obviously HMRC would be interested in your tax affairs!
How would you get references and/or a P45 to start a new job?
Unless you do a 'Reggie Perrin', when nobody recognised him, it is very hard to disappear completely, there are usually some traces left behind - remember the ex MP John Stonehouse who was found in Australia (although Lord Lucan has never been found, dead or alive!).
The Salvation Army are also very good at tracing people if their names haven't been changed.