Pretty standard to get a 2 hour slot and check out within that time span. I've been able to secure weekly Tesco deliveries and would reiterate what a poster stated up thread, they don't take the money until it's delivered, but I imagine that's the same for all. I'm pretty sure that's how it worked for Ocado, those were the days sigh! when their slots were easy to obtain.
I think Tesco, not a supermarket of choice before the lockdown, have offered a good on line service, although I don't use if for everything, my local Marks have better quality produce, particularly for meat and fish. I went there today at 9.30ish a good time for my local store, very quiet no queues and their fresh fruit and veg counters were heavily laden with the very best. Also managed to get plain flour, baking powder and golden caster sugar, all unavailable at Tescos on line at the moment.