UKC

I'm not a cyclist but do occasionally get Lime bikes in London...any recommendations for a lightweight helmet which won't be too onerous to carry around?

I'd start at a big Decathlon and see if you can find one of theirs that fits your head well. If so you'll get more for your money, or pay less than you would for a big brand one. They weigh a couple of hundred grams so I don't find any helmet onerous to carry around. What type of bag do you use when you ride bikes? I'm sure you'll find somewhere to clip and some rucksacks like Osprey ones often have a plastic widget on elastic to hold your helmet. 

I wear one of these when I’m cycling in London. 

To the disliker/s…I actually deliver office supplies. 

Haute Route Helmet | Mammut https://share.google/NyrdvUuAP6pUF0XhW

Another option is to get something like this which you can use for climbing too. Actually looks like it gives better protection than most cycling helmets tbh

The chap is asking for a bike helmet for occasional use on a hire bike around London, and you're suggesting he gets an 120 quid ski mountaineering lid (that presumably to pass the alpine skiing regs has very few vents in it)?

My decent Decathlon helmet is 100 grams lighter and cost a quarter of that cost. 

I've a Salomon equivalent helmet which is rated for skiing, cycling & climbing. I bought it mostly for skiing & its great for that. But although very comfortable to wear, it wouldn't want to wear it cycling or climbing in summer as it just doesn't have enough ventilation. Even with a well vented cycling helmet my head was sweating profusely yesterday afternoon cycling when it was in the high 20s, would have been awful in my skiing helmet.

I'm always wryly amused when people baulk at the cost of climbing helmets. Point them towards a boutique bike shop

It's an option if he also needed a climbing helmet.  As you say you can pick up cheap lids for cycling all over the place.  

Thanks, for these purposes I am looking for the cheap(er) just cycling option

No view on them, but look at the foldable bike helmets - maybe good if carrying?

 But what you have to remember is that cheap (and IMO, horrid) bike helmets also exist, climbing helmets don’t really fall out of the £70-130 range. 

I beg to differ: https://www.decathlon.co.uk/p/climbing-and-mountaineering-helmet-rock-blue/...

Personally, I never got on with Decathlon helmets - they always felt overly big for me. 

I’ve had a Specialized Align for 5 years, and have just upgraded to the new version (with MIPS).  Under 40quid and avail. In lots of colours, including white which I feel is one of the most visible options. 

Lidle have some in their centre aisle, cheap cheerful and lightweight (although still with an integral rear LED.)

I think with a lot of these things it depends whether a particular model fits your particular head

(I have a BD Capitan that fir some reason just doesn't sit comfortably at all.)

I did think of linking to this rather snazzy looking inflatable helmet which would have ticked the boxes aside from cost at £350.

I am curious with the growth in hire bikes whether some better options will appear with folding/inflatable helmets since does seem an obvious market.