Thetford Aqua-Kem Green?
Why are you looking at alternatives? Cost, effectiveness or environmental concern.
Several years ago, Thetford Blue was reformulated to omit formaldehyde as it's cancerous, toxic and banned throughout Europe. Thetford Green has never contained formaldehyde and is more environmentally-friendly than Blue but not as effective.
I believe Elsan Blue still contains formaldehyde but is legal to use in the UK.
Fenwick's Top 'n' Tail is formaldehyde free, environmentally-friendly and can be used in the flush tank as well as the main tank. I shall be trying this for the first time next trip so I don't yet know it's cost effectiveness.