Can you ask for extras when you order from the machine?
You can; just click "customise" on the kiosk and you can add/take anything off.
How much are they?
They vary slightly store to store due to corporate and franchised stores, but at mine they're £3.09 on their own or £4.59 in a medium meal (£4.99 for large, and of course drinks like milkshakes, McCafe and lattes cost extra).
If you want extra cheese or tartar sauce, it's an extra 20p (which will go through on the till as a select dip as there's no other button for it).
Haven't seen one in years... They're not on the menu on any I've been to?
They're available in every McDonalds in the UK. With the exception of test products, all McDonalds are uniform and sell exactly the same items.
I do get annoyed that there don't seem to be any discount vouchers for them (most that I see are for a Big Mac or chicken sandwich). If I wanted a Big Mac and fries I could get one for 1.99 almost any time, but FoF and fries is always about twice that, as there's never any offers going.
All the bus tickets that offer £1.99 Burger and fries give you a choice of Big Mac, Chicken Sandwich or Filet o Fish. They're also on the coupon pages given out and you can get them via the Food for Thought process - all £1.99.
Love them but they are tiny compared to other sandwiches I reckon.
It's a reg bun (the same bun used in Cheeseburgers, Hamburgers and Double Cheeseburgers), just steamed. So it is slightly smaller, yes.