I am from the UK also, but due to the lack of clarity regarding laser laws here, I now only buy lab (mains powered devices) unless it is rare like the 589nm lasers which are still ridiculously expensive in Lab form. As far as I am aware, I have found no laws to suggest illegality. The main issues surround the usage of the device, in which it should be obvious that, if pointing a hazardous laser at people, animals and means of transport, that it would be a irresponsible and punishable decision. The other factor that you have to look out for; and is the sole reason why I do not buy portables any more, is that it is under the discretion of the police officer at the time, to whether or not to arrest you, based on his judgement on whether the laser is considered a dangerous weapon in the same criteria as firearms, tasers, and blades. Also whenever transporting the laser around, remove the batteries. Having the batteries in is the same as having bullets in a gun. I suggests that the laser was intended for use. It won't do much in your defense, but you can then prove that you hadn't intended to misuse the laser.
Luckily I do not believe the laser is truly a 5w as these are usually obtained through specific sellers of which mostly are on this site. Also if it is a 5W it would burn you instantly and would not have to measure how long it takes. However, it is clearly in between 1.25-1.5W and is most likely a cheap chinese model. This means that the laser is incredibly dangerous, so I hope you are taking precautions.