Thanks for the tip. I didn't take the tube/bus over the last two trips to London since I had a bike both times.
For the benefit of other people travelling with a folder in the UK:
1. A bag is really required on the Eurostar. They won't even let you go through the metal detector without the bike being folded and bagged
2. No bag was required on the local trains I took (Virgin, CrossCountry, etc.)
3. On Megabus and NationalExpress coaches, it's up to chance: Some drivers know a bag is required and might not let you board until you bag the bike, some will warn you but let you board anyway, and some won't say a thing because they either don't care or see so few folders anyway that they aren't even aware that a bag is required :-)