Hi Albain,
As G has said, I'd be very surprised of such an amphibian could be leased anywhere, let alone the UK. I cannot speak for the UK, but you'd find that for the few amphibious Cessnas you can rent in Canada, the limitations on where the aircraft can fly, and who flies it to be so restrictive as to make the effort pointless. It's basically an insurance situation.
For your reference, I, with 600 hours of floatplane experience, and a further 500 hours of amphibian time, and owning a non Cessna amphibian, am insured on my friend's 182 amphib, and the insurance premium for that exceeds $10,000 per year. The insurers know that anywhere you land on the water is not an airport, and often away from aircraft support and infrastructure, so any problem is a big one, and, the risk of landing with the wheels selected to the wrong position is very high. The insurance premiums are set accordingly, and that pretty well makes renting them out impossible to insure. For your reference, that 182 amphib is based in Norway, and the operating cost is budgeted at E500 per hour.
In Canada, renting a floatplane is possible, though not common. Generally, there is a lot of "process" to go through to get to take the rental floatplane to another body of water. You do not just hop in it and go.
The best plan to use an amphibian is to purchase it, and then sell it when you're finished using it. In addition to a Cessna amphibian, also consider a Lake Amphibian. They are actually more safe, when flown by a well trained pilot, and less expensive to buy, though maintenance is not cheap on them.