Comfortable is such a wishy washy term.
Somebody who has little urge for material wealth may happily live on a sub 30k salary (currently anyway), certainly I think that defines much of my family. Most of them rarely go on holiday, they have older cars, don't go out much and don't have expensive tastes.
That might sound stupidly boring to many people, but it's just the way they were raised and they are all happy with their lot.
Currently I would say I am 'just' about comfortable on a 13.5k pa PhD stipend. I take occasional short breaks around Europe, live in a nice enough flat/apartment that is rented, own and run a car, eat well and go out most weekends. I don't however currently have any savings or "stability" should something terrible happen.
I expect that as I graduate and have a better idea of where I am to settle semi-permanently, I will buy a house, want a mortgage, proper pension plans, perhaps take proper holidays due to having specific time to take off rather than the student "ok im bored lets go somewhere next weekend" attitude, and as such what I consider to be "comfortable" will increase.
To say that somebody in the UK can't be comfortable on anything less than 100k is frankly ridiculous, a large proportion of the population live, in their minds, very comfortably on ~30k and then the pension this brings their entire life. They don't have brand new luxury cars, they may shop at Tesco instead of Waitrose, but for them, it's enough and that is 100% fair enough. Clearly for a select few, perhaps those who have been brought up to be used to more or perhaps have got a taste for it, then clearly 30k wont be enough and I imagine this is the driving force that allows those people to work/connive that much harder to climb the social and economic ladders!
EDIT: Just to add PMKeates example above this post is exactly what I mean. For most people in the UK, living in central London in a nice house is something they wouldn't even consider to be something they could ever aim for, so they don't and to be frank for many it's undesirable. I have NO inclination to massively pay over the odds purely to live in the capital of a landmass that is so tiny to begin with. Sure London is "where it's at" but boy do you pay for the privilege! If that's something you desire, then that's great. The same can be said for first class flights, expensive watches, top spec brand new luxury cars, sharp bespoke suits, eating in fine dining places instead of your local independents, the list is endless. As has been said, for the individual these things may seem to be necessities of life as that is what they are used to, but they are not a requirement for comfortable living, not even close.
When you earn so little that upon needing urgent medication, you have to wait for a few weeks for the next pay day before you can even afford NHS rates, then that's not living comfortably... Unless you have experienced it or witnessed it, you have no context and arguably no understanding.