I've done a bit of research and found some interesting anecdotes, but not much that's useful in your case. I'll make a couple of assumptions - please let me know in the comments if any are incorrect and I'll change this answer accordingly.
- The 50mm of limestone you're thinking of putting on top of the native clay soil is crushed, with sharpish edges, as opposed to rounded edges (such as you'd see with pea gravel).
- Equipment of various kinds may be run over the limestone layer during construction on the site.
- Because of the heavy clay on-site, drainage is a concern.
Assuming my assumptions are correct, then you may be setting yourself up for a very poorly drained lawn. Why? Because the limestone will compact, either from being driven over or simply over time. As it compacts, the limestone knits together to form a thin layer of what is essentially a driveway/car park. This will act as an semi-permeable or impermeable barrier to water soaking into the ground from above, creating a perched water table.
Now, this is not necessarily bad. In the US, golf courses intentionally create perched water tables to aid in both drainage and water retention for their greens. In these cases, the water table is perched by using sand over a gravel base, which is similar to but of course not identical to your case. It is also something that requires expertise to create. See here and here for brief discussions about this. The difference in pore sizes between the sand and gravel in this setup are not, however, that significant (both are considered to be "coarse" materials, but one is much coarser than the other).
In your case, the topsoil will have medium-sized pores at best, so you're looking at a three-layer cake of medium-very coarse-extremely fine pore sizes. This generally leads to poor drainage as the water hits the first pore differential (topsoil to gravel), slows and then slows further when it hits the gravel-clay interface. As the water slows, it backs up into the topsoil. If the topsoil is not higher than the depth this water reaches, then you're looking at, essentially, a swamp instead of a lawn until the water finally makes it through the three layers.
You indicate that you're planning on putting at least six inches of topsoil on top of the gravel. This is probably too little, as this soil will compact to about three-four inches. I've seen recommendations of at least a foot of topsoil in similar situations, which was expected to compact down to about six-eight inches.
So, would I put the gravel in? Absolutely not. To improve drainage after the construction, I would first use deep-tine aeration (most often used for sports fields and new construction on former farm fields) to break up the clay's compaction down to about a foot. I'd then use regular aeration to break up the surface clay even more. Following that, I'd lay down a mix of compost and topsoil at least eight inches deep (to allow for the inevitable compaction as the compost decays and the normal compaction of topsoil). I'd then seed or sod, as appropriate.
As an alternative, I might lay down two-four inches of compost directly onto the clay (after aeration) and rotovate that in before adding the topsoil-compost mix. The more organic material in clay, the better-draining it becomes.
For ongoing maintenance, I'd use compost instead of fertilizer yearly - again, to build up the organic matter in the soil.
Perched water tables were already excellently discussed in this forum in a post regarding container planting.