It depends on where in the world issued your driving license.
On the Gov.UK website there is a Driving License Exchange tool. If you follow that answering it with your situation, it will tell you if you can exchange your license, or if you'll have to re-sit.
For example, if you are now a resident of the UK, and hold a full Canadian car driving license you can exchange it, if you fill out the right forms and pay the fee, up to 5 years after moving. For an EU license it's similar, with you being able to exchange (up until you are 70 years old, or after 3 years into entering the country, whichever is longer).
However, if it's not from one of the right list of countries, then you can only drive on the license for 12 months, and after that you'll have to pass the theory test then practical test to get a brand new UK license.
(I think that if you can't exchange your license, then before the 12 months are up you can apply for a provisional UK one, prepare for the test, but keep driving on your foreign license without L-plates, but you probably want to ring the DVLA to double check on that one...)