I guess you could, but that might stop the roots getting well down. Were you thinking it would be a waste of compost? You wouldn’t lose too much.

I also fill half the pot with polystyrene chunks and then just put compost/soil over the top. I always find, when emptying the pots, many Woodlice within the polystyrene and not in the soil, which can only be a good thing!

There is a contention that as long as you use reasonably open compost and good drainage holes you don't need to use crocks or polystyrene for drainage in pots. Also be aware that polystyrene is not biodegradable and will be around for hundreds of years and you will keep finding it laying about in the garden should it escape your pots.

Best to just put soil on top,without a membrane, so roots can go down to wherever the soil is. But that looks like too much polystyrene, seems more than half full with it, I'd reduce that amount by half. If you want to lighten the pot more, mix the compost with vermiculite or perlite.

Thanks guys, will take some out.

what are you putting in the pot? Summer bedding in which case you wont need that much topsoil though I'd use vermiculite to lighten the pot.
if it is a long term planting I would reduce the polystyrene and add more compost.

I put a few rocks at the bottom to add some weight, then fill the whole pot to the top with the best potting compost I can find. Then place the planter where it's going to be and never touch it again until October. This way it won't dry out so quickly and I can go away for a few days without worrying about it. Secondly, It's weighed down with the the rocks because of the violent storms we are prone to get and hopefully won't end up in the neighbor's yard.