It's going to depend on what has caused the light to come on. It could be something as straightforward as an EGR valve sticking or a momentary rich condition which wouldn't cause much of an issue. It could be something more serious resulting in the engine running in LOS, limp mode. This would result in poor performance and terrible economy. Best to get the fault codes read to find out what's wrong. It will fail the MOT with the light on so it'll need fixing before the MOT is due. You can pick up a half decent code reader on ebay for about £20 so it may be worthwhile getting one and reading the codes and resetting the light yourself.