Putting an Australian sea plane rating onto a UK PPL
If you want it on your CASA license then go for it, but if you want it on your CAA licence then you'd be better off doing the training back in the UK instead.
As i understand it your Australian ICAO training will be recognised in the UK but you may have to do the UK written examination and possibly the UK float plane skill test to have the rating added to you UK PPL. The float plane exams can be taken at a DTO or ATO that provides the training in the UK. I would suggest contacting On-Track Aviation for good specialist advice, I'm sure they will be helpful.