If the U.K. hadn’t declared war on Germany in WWII after Hitler invaded Poland, means that Hitler would either invade Britain, or bomb it to smithereens, so he could continue his plan of Lebensraum. Anyone allowed to remain alive in Britain would be used for slave work, until they were starved or worked to death.
Yes, it would have been terrible, but please let‘s not exaggerate: the plans concerning a German occupation of Britain were not well fleshed out. While, indeed blacklists had been prepared for individuals to be liquidated after the conquest, not much attention had been paid to the fate of the average Briton.
Yes, there was some talk about hiring or conscripting British labourers for the war effort, but hardly more. Possibly, the British Fascists would have seen a resurgence and collaborated with the occupying power, possibly even forming a kind of provisional collaborative government - as we have seen in several mainland European countries.
But Hitler did indeed see the British as racial equals - maybe even superior to Germans. He had a little Anglophile within him 🙂 Strange but true.