I would first like to start off and say this is a fascinating discussion. I cycle, i cannot drive as i do not possess a drivers licence and there are a lot of interesting points brought up in this thread.
When i was predominantly living in London, except for on the canals and the central cycle superhighway ( an excellent cycling artery through C. London - somewhere where you can witness cycle congestion due to the huge volumes of cyclists) in London, i did not see many other people cycling, why? Non existant infrastructure, its so dangerous and quite terrifying to cycle in London sometimes, but is often the most time efficient way to get around, and as others have said is good at least for your physical health.
Having been forced to cycle into my job at Tesco's for a few weeks while at uni when my car's turbo (it was a diesel car, so don't get any ideas about me!) kicked the bucket, it was my own housemates who pointed out that I was suddenly way more short-tempered than usual during that time period; my friends from lectures also indicated that I was zoning out of group sessions way more often than normal. While I noticed neither, I did notice a dramatic decrease in my energy and vigour at work. All 3 noticed I'd gone "back to normal" after getting my car back.
Whilst i don't agree with a lot of the sentiment of this poster, they do raise a rather good point, the notion that cycling is a stress reliever and mood uplifter is not universal for everyone. I'd say poor infrastructure and dangerous conditions have a lot to answer for here. By my own admission i find when i have to cycle on roads in London, i tense up and do find myself having to get internally quite agressive and ascerting my space in the road, for my own safety, and i would not say that is good for my mental health. But if you are for example cycling across Shepherds Bush Green on your bike, and over the Holland Park roundabout (better cycling infrastructure desperately needed here) you are going to struggle to do that if you do not ascert your space in the road and in particular going in the middle lanes to avoid clashing with left turning traffic going straight ahead. On those rides, you become quite the hot head, not to mention sweaty, because of the stress entailed there, so i can empathise with this posters sentiment that cycling to work made them feel a lot worse.
However in Umeå, northern Sweden where i am for university the cycle infrastructure, all dating back from the 1960s is so much better, probably the best i've eveperienced where it is mostly completely segregated from the road such that i very rarely have to cycle on the road, and then it is a delight to cycle. Then it is stressfree, then it feels safe. And yes you will find people quite regularly cycling in the ice and snow there when it is minus 10. Granted there are days it is so icy it is not safe to cycle at all, even walking or driving is hazardous, but then thats usually when its around 0 degrees C.
So when people moan about unused cycling infrastructure, it does break my heart a bit, because theres many times i've wished for better cycling infrastructure. Another london example, but i know the cycle lanes on the Chiswick High Road were not without the controversy removing the bus lanes and increasing vehicular congestion, but as a cyclist those bike lanes are wonderful and they are also very well used, by all ages, you see a lot of families cycling alomg there that never would have prior to the creation if those cycle lanes. So when done right, it can have a huge impact for cycling, then that the UK in general has a very backwards attitude to cyclimg and cyclists, with many car drivers actively hating on cyclists (there is some evidence of that one here) might explain why there is so much congestion, as too many are wedded to their cars and believe it is their god-given right to drive and trumps everyone elses rights to mobility in cities, despite cars being one of the least universally accessible mobility solutions.
To round off as a cyclist in London id say the following places are crying out for some dedicated segregated cycle infrastructure, and no, white lines on the road, does not cut it in my opinion.
Savoy Circus, East Acton, crossing the A40 - horrendous crossing as a cyclist, and whilst there are pedestrian subways either side these require dismounting with the bike (significant time penalty) and are not at all safe - heard of far too many muggings and violent assaults in those subways in East Acton that i avoid them at all costs.
Uxbridge Road - theres some limted bike lanes around Acton Town centre and Ealimg Common but there could really do with being a segregated bike lane for more of it, especially by Ealing Broadway.
Holland Park from Notting Hill Gate down to Goldhawk road/Uxbridge Road includimg Holland Park roundabout and shepherds Bush green, lethal as a cyclist.
Old Oak Lane from North Acton (perhaps even extend down to Acton Town Centre via Acton Main Line - following the bus 266) to Harlesden town centre neatly intersecting the Grand Union Canal half-way would create a good safe north south cycle link across west London and into Harlesden which like most of Brent is virtually devoid of bike infrastructure.
Kensal Rise, in particular Chaymberlayne Road, could do with some segregated bike lanes to make things safer there what with all the buses and cars there, and that would connect into the intricate network of back roads either side that provide relatively safe routes for cyclists to traverse Brent.
And as for bikes on trains in the UK, anything with a cupboard where you have to hang your bike up (voyagers, IETs) is completely impracticle - handlebars are often too wide so a cupboard that claims to take 2 bikes only fits one, for older people like my mother, she doesnt have the strength to lift the bike in place - but these were clearly designed by people who do not cycle. The Thameslink 700s, or even the elderly 156s (granted its hard to manouvre the bike through the narrow vestibule on those) demonstrate how bike spaces on trains should be. And yes i do sometimes like to take my bike on the train to do longer rides in the countryside where you can relax and enjoy cycling, rather than getting all stressed and sweaty in London traffic.