use aluminium layer on the current concrete slab, to reflect the heat back up.
Hah. No, alas. If that worked then you wouldn't burn yourself picking up a stainless steel pan full of boiling water by grasping the sides because the mirror surface would be reflecting the heat back into the pan
Shiny surfaces reflect radiant heat, not conducted heat. Your screed slab with UFH pipes in will conduct heat to the aluminium it is in direct contact with; no heat "reflection back up" will occur. It's also likely that the aluminium will corrode and possible that it will subsequently damage the screed chemically. An aluminium layer on top of a slab with screed poured on top of the Alu would do absolutely nothing to prevent conducted heat reaching the slab. A floor-on-floor of a network of battens with aluminium spreader trays between and a chipboard floorboarding over would do next to nothing to stop heat reaching the slab, the only insulation in that direction being offered by the air gap under the spreader plates
Your UFH pipes should perhaps (depending on the use pattern of the room, see below) have been clipped to the insulation below the slab and the slab poured over them. Now they should be clipped to the slab or to something clipped to the slab (clips, clip rails, egg trays) or heavy enough to stay put (reinforcement mesh, but take care not to stand on the pipes tied to the mesh) or to another layer of insulation barriered with a plastic sheet so screed can't get underneath it, to stop them floating when the screed is poured. Even if they were full of water (and it's recommended to pressurise them before pouring but doesn't have to be with water), without clipping they would float in the screed..
So long as your room is often used, It doesn't matter about heating the slab because on/off/on/off is how radiators are run, not UFH. You use UFH to heat a well insulated slab to some low temperature like 25 degrees (more if you have an insulative floor covering like carpet) inside a well insulated room. The room then maintains a temperature like 22, and you pay to constantly heat the slab a small amount as it loses a small amount of heat to warming the room. The insulation below the slab, which is hopefully quite a thick layer like 100mm+ (of PIR or equiv) inhibits heat loss into the ground so the heat loss from the slab (that you pay to replace) goes to heating the room. Insulate your room walls well, and ensure that poured screed is insulated from the walls (run the wall insulation right down to the under slab insulation) and it won't matter that the slab takes 5h to get up to temperature because once there it stays there with minimal ongoing input
If the room is used only occasionally, UFH may not be the best choice but you can make a faster responding UFH that you can blast on and off like a radiator with eg a grooved cement boards layer on top of a thick layer of insulation
UFH needs designing properly for the use pattern of the room, ideally before slabs are poured.