The 16-port Gen2 is a very poor device compared to the Gen 1. The Gen 1 has 16 PoE ports whereas the Gen 2 has 8 and a pointless tiny display. The Gen 2 24-Port is a better device with 16 PoE ports and a silent cooling solution. I’d still pick the Gen 1 16-port over either Gen 2 device. It’s only with the Gen 2 24-port Pro that you get the full 24 ports PoE (16 PoE+ and 8 PoE++) and with that you pay for 2 10Gbps SFP+ ports. But that truly blows your budget at £650.
As for heat and dust in the loft, most dust is dead skin cells so if you stay out of the loft, there will be very little dust up there. Heat, yes, there will be a higher than normal temperature on very sunny days but the UniFi line are rated to operate in ambient temperatures up to 40C so you’ll have no trouble in a UK loft, even in the hottest summers.