I hadn't seen this before although it seems like a pretty straight forward extension from Carrier.  just remvoe the carrier deck and install a pool.  Would be a lot of neat float planes that could participate as well.
Link to the Barton Flying club topic. 

https://controlline.org.uk/phpBB2/viewtopic.php?t=13958

Can't tell how popular it is, there just seem to be a couple of entries so maybe it is/was a trial idea.  I suspect most would say it wouldn't be worth it considering the limited number of carrier entries at contests anyway.

I will say this, it does look like the UK and New Zealand flyers still follow the pre-slider rules which is interesting.