No one knows and it is impossible to predict.
Any sensible government won't relax the control until it has mass testing in place, so it can accurately monitor the number of new infections in each region. If they see the rate of infections increase, they can then decide to wait and see if this is a trend and if so, reimpose the controls.
In other countries, the governments started by allowing small shops to open and in some cases also hair salons. Although in Germany, small shops opened last week and hair salons open on Monday - with a lot of restrictions. But the rate of infection that was falling before last week, has slowed. If this starts to increase after hair salons and bigger shops open on Monday, they may stop them again.
The UK doesn't have mass testing in place yet and it is unlikely to meet the target of 100,000 tests a day before end of this month. Even that is not enough and only covers essential workers (there are 1.5M NHS alone).
Personally, I can't see the difference between allowing hair salons to open, or nail/beauty salons to open. The risk seems to me the same. All are close contact services over 1 hour or more. All will require masks for both provider and clients, 2m distance between clients, etc. But it seems that hair salons are considered to be more essential by the politicians.