We're about to order topsoil and turf. Should topsoil be compacted first, or just leave it loose?
Well the answer depends on exactly what preparation you've already done to the turf bed. If you dug the area over and removed all weeds, especially perennial ones, that should then be raked level and left to settle. If you're then adding topsoil, spread it over, then walk all over it closely, using your heels, to find any soft spots, then relevel with a rake or a long board dragged over the area, so that you know its flat all over. It's not a good idea to use a heavy roller, you don't want to compress the soil deeply, you just need to make sure the soil is even and flat, without dips and hollows. This is necessary not only for good, complete contact in terms of the roots of each turf, but also for having a flat, even lawn over time.
If you can, wait a few days between spreading the topsoil and actually turfing,so ordering the turf to arrive a few days later would be best. The reason for this is that it gives the area a chance to settle and consolidate; any weed seeds or pieces present in the topsoil should produce growth quite quickly at this time of year, and you'll be able to remove those before turfing. Topsoil often contains quite a few largeish stones and pebbles, and they will need removing prior to laying turf - small or tiny bits of stone are okay. When your turf is delivered, you need to lay it pretty quickly, preferably within 24 hours, but certainly within 2 days, so having time to screen and spread the topsoil without the pressure of a pile of turf sitting waiting to go down makes things easier and you'll do a better job of it. If you can't get it down within two days, you need to unroll the turves and keep them watered till you're able to finish.
The top two inches of the turf bed should be friable and relatively loose, to enable the turf roots to penetrate easily, so rake it up just before laying - it should also be damp, not dust dry, and given the weather we have in the UK at the moment, should it continue, that might mean you have to water it before laying the turf.