Perhaps the Ross alternative was under way at the time and thought to be the better option?
There are at least two photos of one in the water (the same one), so I imagine that was about how far that project progressed.

Guess this was only a small scale model!

Never heard of a BN-2 on floats.
The sim community shows such conversions.
Also the rc community (here originates the 3-view draw)
If you make a quick google you can find this:
“Delivery Flights – Piston/Turbo-props –
Pilatus Britten-Norman BN-2B Islander, G-HEBR, Night stop between 29-30.11.10. c /n 2132, …. Equipped with floats.”
http://www.aviationinmalta.com/CivilGeneralAviation/Deliveries/DeliveryFlightsPistonTurboprops/tabid/430/language/en-US/Default.aspx

But using this spotter site you will only find Cessna 206 and 208 on floats –
and
Pilatus Britten-Norman BN-2B Islander, G-HEBR, Night stop between 29-30.11.10, c/n 2132, previously registered to Hebridean Air Services. Previous reg G-BJOP.

In an article dating back to 2005 they mentioned this:
“Britten and Norman left the business in 1974, but development continued with a 6,600-pound-mtow, amphibious BN-2A-30. Twin floats were attached to the undercarriage legs and the fuselage incorporated retractable landing gear.”
http://www.ainonline.com/ain-and-ainalerts/aviation-international-news/single-publication-story/browse/0/article/britten-norman-islander-celebrates-40th-anniversary-2278/?no_cache=1&tx_ttnews%5Bstory_pointer%5D=11&tx_ttnews%5Bmode%5D=1

And in 2006 the following:
“Britten-Norman’s Group Chief Executive, William Hynett, said “We are really pleased to have Jeremy Liber on board. Jeremy has some exciting plans that will bring a new impetus to the Islander and Defender aircraft, including cockpit and instrumentation upgrades as well as additional airframe features which we hope will extend to a floatplane variant.”
http://www.worldsecurity-index.com/pressdet.php?id=874

You can find this from the late 90s – thoughts of an Islander Amphibian from Ross Aircraft Company (RAC)
Their website is long gone (heeley.demon.co.uk/)
http://kheeley.tripod.com/rn.htm#SPEC
http://kheeley.tripod.com/rn_sub2.htm

G-ABVP is the Monospar S.T.4 – not a S.T.10
http://www.goldenyears.ukf.net/reg_G-A3.htm

“This week Eve Branson, mother of Virgin Atlantic tycoon Richard Branson, writes exclusively for Mirror Woman about how she trained to be a glider pilot during the Second World War before working as an air hostess for British South American Airways – and how her son inherited her adventurous spirit”
http://www.thefreelibrary.com/I+told+my+son+Richard+how+to+be+a+high+flyer%3b+BRANSON’S+MUM+ON+HOW+…-a061304478

…or the WH453 at BWCM
http://www.bcwm.org.uk/html/gloster_meteor_f_8_wh453.html

Brooklands?

Aug 8, 2010
http://airfields.fotopic.net/p66533805.html

Interesting to follow the development of the outdoor collection on GE (30°21’11.77″N 87°18’17.04″W) between Jan 31, 1994 and Apr 25, 2010