I don't possess a fixed swimming pool, but I can't see a garden swimming pool being of much practical use, in our climate. It would seem to require a lot of maintenance work - for very little return.
I had a large above-ground inflatable pool, once. One Summer it was used on just one occasion. It was a waste of money, and space. I got rid of it and put the area to better use.
Converting a legacy swimming pool to a more natural pond is an excellent idea.
On our previous BBC forum, one of the forum members had actually done this conversion, and had made a wonderful job of it. They posted some photos. I think they spent quite a lot making their conversion. But huge expense isn't absolutely necessary.
The two main elements that you need are - something shallow to stand some plants on, and also some way for any creatures to climb out. Whatever supports the plants, together with the plants, would suffice for that. Something like a raft, made of wood, might work; I'm not sure. Or possibly something like a couple of packing crates.
...What I was hoping to do was chuck in a load of bricks, stones ... I thought I might add a filter ...
I would not have filled a pool with 'a load of bricks and stones'. Retaining depth is useful to prevent the bottom from freezing. And a filter is quite unnecessary for a natural pond.
Bear in mind too that it's very difficult to revert a pool back to an ordinary garden simply by filling it in, because there is no drainage, and water cannot drain away.