Whilst you are not legally obliged to wear a bike helmet in the UK, you definitely should!
There are plenty of strong arguments for not wearing helmets: Cycling great, and all-round good egg, Chris Boardman often points to a couple of facts:
No argument from me on either of these. However (1) is a classic cause-effect trap: It doesn't imply that if the the Dutch all started wearing helmets head injuries would go up.
Similarly with (2): I agree that if a greater proportion of the population started riding, the death rate would go down even if none of them wore helmets! BUT - we are not talking about the life expectancy of the general population and laws of large numbers; we are talking about YOU, and you already receive the health benefits of cycling.
Of course, I'm a hypocrite as I sometimes go riding without without a helmet. However, this doesn't mean it's a good idea; it just means that sometimes I'm an idiot!
This is a selfie by 41 year old, mother of 2, Carmen Greenway. Seconds later she hit a rough patch in the road, came off the bike, and fractured her skull. Carmen died six days later in hospital. Her husband and family released the photo in the hope that more cyclists would wear helmets.