Accurate British English term for an oblong deck from shore out into a lake where you tie your rowing boat

This is a typical image of the structure in question:

There are also some variations, shown in this Google image search.

But I'm after the often not very wide, some 20-30 feet long wood construction a family builds (or hires someone to build...) for their summerhouse. For the kids to get out a bit so they can dive in the water or do some fishing, and to tie their small rowing-boat or motorboat to (not yacht). It's rarely the big, square deck with chairs to sit and look over the lake; usually no more than 5-6 feet wide; just for walking out to the boat or the front of it to jump in the water.

What I've come up with so far is landing-stage, but a Google Images search for this brings up big entertainment piers and some other things, so I'm unsure.

Purpose/context: This is for a translation into British English of a text concerning construction of decks, front steps, "landing-stages", gates and other objects using the same product: pressure-impregnated lumber.

I'm after the synonym that you think conjures up the image at the very top in the mind of British readers. It can be an accurate term or collective term; as long as it's distinct from "deck".

If not a construction term, at least a term for the construction, as opposed to "object for doing x".

I need to describe the construction/object itself, not a place or a generic word pertaining to function (without it ALSO meaning the concrete object itself). Using a term for the construction in which poles are driven into the lake bed is fine, although some float they have poles with rings to hold the construction in place horizontally.

Which one of pier, jetty, or landing(-stage) is most apt? Which of these can one person order deck planks, joists, and screws for, and build using normal tools and apply oil on with a paintbrush? This is what I'm after.

The word that you are looking for is pier. You can have long ones, short ones, large ones and small ones. Its dictionary definition reads like so:

• A raised platform built from the shore out over water, supported on piles; used to secure, or provide access to shipping; a jetty.
• A similar structure, especially at a seaside resort, used to provide entertainment.

An image search for kids jumping off them should bring up results which should confirm this. I believe that dock can also be used. But it is technically the area where a ship or boat is ... docked, rather than the structure itself which is a pier. There are similar distinctions for words such as jetty, wharf, and perhaps, even quay.

FWIW, Wikipedia also has articles for each of these structures.

I'd call this a jetty myself. Pier isn't wrong, but tends to imply a more magnificent structure than that shown.