Buying extension tickets from machines provides me with hours and hours of harmless amusement. At present, as the previous poster said, all the helpful Southern machines that were the easiest to do this with are being upgraded with 'computer says no' machines, ie S&Bs. SWT modified their machines earlier this year to allow Boundary Zone x as the origin but when one tries to buy one either the machine software collapses altogether (seriously!) or it simply comes back with 'no fares found'. Many of the ex-FCC machines were OK, but are now being 'upgraded' with the S&Bs. However, I have discovered that the VTEC machines at KGX have recently been reset to allow Boundary fares and I even managed to buy one from them last week. The FGW machines at Paddington, amazingly do offer this option (this TOC is hardly the byword for making things easier on ticketing) but the CH machines at MYB do not and the queues at the ticket office are often very bad. I suppose I would like to think there is some rhyme or reason for this situation, eg. discouraging fare evasion. But I think, rather, it is simply a question of the cost of the TVM software and TOCs willingness to try and be helpful (Southern management).
More seriously, I wish someone would take up a 'collective action' with DfT about this situation and Penalty Fares. The introduction of Penalty Fares was always predicated on having a suitable range of tickets to buy from machines or a ticket office. But the reality is this is not happening. Although many stations in the London area do have ticket offices, it is clear that (presumably because they make such few issues) they are not always open when they should be and are, effectively, unstaffed (just go an survey some of those in the outer zones to see this). Perhaps I can just pay at the other end in such situations. More often when this happens I just don't make the journey and do something else instead. TOCs should be required either to have operable TVMs that sell Boundary tickets or reintroduce Permit to Travel machines (still out there in a few places). The current PF regime (ie you are PFed because we can't sell you the right ticket) is simply not working well enough.