Great to see you at the show, good discussion point.
If you consider that most deck light fitting are dropped in from above the deck and the cables tracked underneath to junction boxes. It is also usual to have access panels near down pipes, inspection covers and for other items under the deck so best to have your connections in this location and you can then access these at any time.
In accordance with part P and the 17th edition, you are required to have access at all times to all electrical connections in any circuit.
In a normal surface fixed decking installation you would have to still take up a number of boards, not to lift the light out of the deck but to get to the connections. In all side fixed decking structures without a removable hatch you are then faced with the same issue.
I suppose on a deck that the designer or client doesnt want a hatch for any reason, perhaps a small section of fascia panel can be fixed as a removable panel and still provide good access to electrical connections.
We have had one project where the planters on the deck have a plinth that is removable for access, the electrical items were in there.
Hope this helps