Yes, Southern do offer some fantastic deals on some Advance singles if you are prepared to travel in the middle of the day or at an obscure time. However if you need to be in London before about 10am on a weekday you will have to pay substantially more than this. When I was taking my mum to UCH earlier this year for appointments it was around £30 each in advance and £53 on the day (we didn't always get a lot of notice).
I went to UCH when the Hammersmith flyover was closed by car and back by train and it took 1/2 the time by train as it did by car (having just missed a train out of Waterloo by 2 minutes). The thing is train travel has much more reliable than it is to go by car, which is why people use it.
The reason (I think) people get annoyed with three and four figure rail fares is that its difficult to see how it encourages one to not use the car when it costs substantially more, when you bear in mind that they are running the train regardless of whether you buy a ticket, and that they have already sold some tickets, do they really need to charge you £100 or more, it doesn't seem relative to the costs involved, especially when you consider that someone on that train is bound to have got some super-duper Advance for just £4.82.
If no one brought tickets they wouldn't be running the train (or a lot fewer). Also why should someone who is prepared pay more than those that are not. Motorway services fuel and food cost a lot more than Tesco, yet they can charge that much because there isn't an alternative, same with the trains.
The problem is there isn't much need to make the trains a lot busier, especially in the south east or during peak hours, because they are fairly busy. Yes we should use trains and buses to get around more, but no there isn't the space for us to all do that on the rail network.
As a taxpayer I am effectively paying twice; once in my taxes which are used to prop up a private-enterprise business and then as an actual fare paying passenger.
As a tax payer you basicly only fund the rail network, not the train services (it costs the government £20 million a year in subsidies to the TOC after payments from the TOC are taken into account - which is a tiny amount for the government in the greater scheme of things). Most people with children at school will be paying more for their children's education than just their taxes (by going to school fates, paying for school trips, etc.).
If you want one and not the other it'll cost a lot more, free train travel for all, your taxes would have to be a lot higher (and you'd never get a seat anyway as EVERYONE would travel by train), no tax payer subsidy (including for the rail network) and your rail fares will have to be higher.
Yet ironically I am in the one group that doesn't get a nationwide Railcard discount on my fares, is this fair? In Germany I could buy an annual BahnCard and receive discounts on tickets (around 25%) which would encourage me to use the trains more as after say 10 trips the card will have paid for itself.
Which is why Railfuture have a campaign to introduce one, however the TOC would say why bother as we have enough people using the trains.
I doubt very much that a boycott would work unless a significant number of people stopped buying tickets. What will probably happen is that a few will try another form of transport and may ironically help with the overcrowding on some peak services!
I don't think even that would happen, a few may try something else for a month or two, but after a while will go back to the trains as they realise how much better it is.