As far as I am aware there are no alternate approved tethering points on the floor in a T5 Cali SE.
It theory you could add some tie down loops to the floor rails but I doubt these will meet the current safety regulations.
The only provision is to either use the Isofix system or the existing lap and diagonal seat belts.
We have a rear facing Britax seat for the 6 month old granddaughter and a forward facing Britax Roma Evolva seat for the 3 1/2 year old granddaughter/4 1/2 year old grandson. The older ones are now well over 15Kg which is the lower weight limit. This seat uses the lap and diagonal as the seat restraint, has its own 5 point harness and is Group 2-3.
Although ideal now, come the time, we can tuck the harness away, or remove it completely from the seat, and just use the van lap and diagonal with this seat. The down side of this is the lap and diagonal will have to be in use all the time when the seat is in the van as it becomes loose when the kids are not in it.
We looked into Isofix, but decided it was way too difficult to find in the Cali to use daily, and, as occasional kiddie transporters, cost was also an issue for us.
Alan