Unfortunately, if you are not a regular customer (or even if you are, but you’ve not tried to make an advance reservation before), you will be unaware that your slot lapses if you do not check out within a given time.
I appreciate it’s too late for this advice now, but what I tend to do, if I want to reserve a future slot, but not necessarily complete the order now, is to fill the basket with as few items as necessary to constitute a valid order (I think there’s a minimum spend of £30 or £40, so make that up of stuff you ALWAYS need).
Then check out, regardless whether you’ve finished or not. This action secures the slot.
You can usually amend the order up to about 11:30 at night, the day before it’s due (don’t know if that differs at all for Christmas). So it doesn’t matter that you’re not really finished. As irrational as it is to check out before you’ve finished, do it anyway!
Sometimes I don’t feel up to doing it all at once (even though it’s all online), so continue to amend the order over the course of several days - still retaining the slot all the time.
Only a few things to watch - you MUST remember that you placed an order without finishing it, and you need to go back and complete it, otherwise all that will turn up is the few bits you ordered to meet the minimum spend. You also MUST check out again, every time you edit it, otherwise you WILL lose the slot - as happened to you. I agree it’s all a bit of a faff, but I suppose it’s the only way they can prevent people hogging slots they then don’t use - leaving them unavailable to customers who would use them. You have to order SOMETHING, before your slot is properly reserved. You can’t speculatively grab one, which you may or may not use.
I personally think they make it pretty clear on the website that your slot is only held for you for a couple of hours, and if you haven’t checked out by then, it’s released.
That’s not to say I don’t sympathise with you that you didn’t realise, for whatever reason. I’m really not sure what Tesco could have done about it at this stage. I’m not surprised they didn’t have any available slots for tomorrow or what remains of today.
I’ll be very surprised if anyone has any delivery slots for tomorrow, that weren’t booked up weeks ago. I’ve recently found it hard to get slots for the “normal” shopping and that was well before Christmas week. I’m wondering if you could actually hire someone to shop for you - a professional, or maybe a neighbour or their teenage offspring, keen to make a bit of extra money for Christmas?
Obviously, you’d have to give them a list, and choose somebody quite responsible, who’s going to pick reasonable substitutes, if they can’t get your first choice.