Hertz in London often do run out completely. A couple of alternatives are renting a van (sounds silly but you can get a good vauxhall caddy two seater with cruise for £25 a day as nobody wants them over Christmas) or go with a challenger/mobile app based rental service like Virtuo who don't seem to realise it's christmas and therefore don't raise their prices at all (3 days at Xmas 2020 is £400 for A group with plat discount from LCY vs. £180 on Virtuo)