It's fine to have with you (to listen to weather reports from the coastguards for example), only transmitting is illegal without your short range certificate but if you are in a life threatening situation, then the authorities tend to prefer saving lives first.
See here Marine Radio (SRC) | Specialist Short Courses | Courses | Learning | Courses & Training | RYA
"The Short Range Certificate is the minimum qualification required by law to control the operation of VHF and VHF Digital Selective Calling (DSC) equipment on any British flagged vessel voluntarily fitted with a radio. This includes both fixed and hand held equipment using International channels.
A radio is an important piece of safety equipment on board and it is vital to understand the correct procedures. Unnecessary transmissions could block out a Mayday distress call."
The above link also shows all of the training centres around the country where you can take the course.