Once a Work Permit has been granted, it (the original document, not simply a copy) should be sent to you by your employer as you will need it in your possession before you enter the UK. All UK work permit holders staying for longer than six months also need to get a visa (entry clearance) before travelling to the UK.
Only if you are not a ‘visa national’ and you have a work permit for less than six months can you simply arrive in the UK with your Work Permit and your passport (which should be valid for at least six months from the date of entry). On presenting these documents to the immigration officer at the port of entry, such individuals should be granted entry for the duration of the work permit.
Everyone else holding a UK work permit will need to apply for entry clearance (a visa) at the nearest British Embassy or Consulate in their home country (or country of normal residence) before coming to the UK.
Because you may be able to obtain free medical treatment from the National Health Service if you are in the UK on a work permit of greater than six months duration, visa applications for longer than six months will often be subject to your passing a medical examination. While visas for under six months can sometimes be issued within 24 or 48 hours, it will usually take at least ten working days to process a visa for more than six months. This process should be faster for nationals of "phase one" countries as above.