Without stepping into the bear pit of debate as to whether helmets are a good or a bad thing, it is highly recommended that you wear one.
The three main factors to consider when buying a helmet are:
Fit

A properly fitted helmet should cover half of your forehead and as much of the back of your head as possible. The internal cradle and straps should be adjusted so that they are tight enough to stop any forward and backward or lateral movement across your head but still be comfortable.
Keeping cool
A helmet should keep your head cool with vents to allow a flow of air across your head. The more vents the cooler your head will be (or colder in winter!). Of course, to create more vents there is, of necessity, less padding between your head and whatever you might crash it into, so a compromise might be what you are looking for. You might also want your helmet to look cool. This is probably where most of your money is going on the higher end helmets.
Cost
Anything between £20 – 200. You will probably get most of the protection benefits you need from a low cost helmet. The higher end helmets will have the very latest innovations in ventilation, aerodynamics, weight saving construction and head cradling fit technology but you should be able to find a lower cost helmet that fits well, keeps your head cool and provides the protection you need.
Features to look out for
- Easy to adjust fit. Ideally a one handed operation so you can make adjustments on the bike.
- Padded inserts for comfort. Preferably removable and washable because your head sweats.
- Light colour/reflective trim. This will increase your visability to other road users. Some helmets even have built in lights.
- Rounded outer shell without edges that might get caught on something in a fall.
- Peak to keep glare of the sun out of your eyes. This can restrict your vision if riding low to the frame, for instance with your hands on the bottom of drop handle bars, which is why most racing helmets have no peak. You can buy helmets with easily removable peaks to cover both options.
Recommendation
On my tandem lejog cycle with my son, I invested in Livall Smart Helmets for both of us. These were new on the market at the time and were attractive to me becasue they had built in lights at the rear, including turn indicator lights operated from a remote switch on the handlebars.
Another benefit was an app feature that allows you can track other Livall Smart Helmet users that are paired to your phone app. This would have been extremely helpful if we had become separated should we have gone on single bikes.
That was a couple of years ago and Livall have been developing and improving, adding new features like crash detection, auto brake lights, a front be seen light, improved 270 degree rear light, a 10 hour battery life and improved ventilation.
They have adopted a kickstarter funding approach which means you can get some good deals for the latest developments, although you do have to wait a few weeks for production. I have just ordered a new helmet for myself for £64, which is pretty cheap for a helmet even without the features for here: https://livall-evo21-smart-helmet-360.kckb.st/cce6dcb6
Oh, and I forgot to mention that they are also one of the more comfortable helmets I have worn.
Where to next?
The most popular pages on the site concern planning your End to End, training for long distance cycling, thinking about the cycling equipment you will need, what you should be eating and drinking whilst cycling and how to create a route for Lands End to John O Groats. Or you can read my own account of cycling End to End to get some idea of what to expect.