If Triumph is banned from the list,
1. Vincent twin.
2. Ariel 4.
3. Brough Superior.
4. Crocker.
5. Velocette Venom.
6. Sunbeam S series twin.

Oh, and don't forget the Riley Cyclecar.

Let's hope there's intelligent life somewhere in space 'cause it's buggar all down here. -- Monte Python