Afternoon tea

I'm not really sure what you're asking, because afternoon tea isn't a thing in the states, so I don't really have an idea what is generally eaten then or how much. But I will say good on you for planning ahead, and look at it this way: if what you've set aside turns out not to be enough (log things to be sure) you can always go for a walk later to make up the difference. Or better yet, you can choose to not be hard on yourself and allow yourself this little "cheat" for the sake of spending an enjoyable afternoon with your mother. One day of letting your hair down is not going to derail your weight loss. It takes a long term failure to log accurately and consistently to do that. So whatever you choose to do, make sure you're not obsessing over it, and just have a good time. And kindly do not go and starve yourself in anticipation before or guilt after. All that will do is lead to binge eating and more guilt. Pace yourself.

ETA: And to your mother, best wishes for a happy birthday:)

It depends if your Afternoon Tea is just Tea with one scone, jam and cream (in which case 800 is a bit high, probably more like 500-600) or if it's multiple cups of Tea (with sugar and milk?), Champagne?, multiple finger sandwiches - which fillings? , mini cakes, pastries and scone(s) - what size? With butter as well as jam and cream? How much jam and cream?. It's impossible to give a calorie estimate without knowing the qty of food you're having and some detail on what it is. Some 'Afternoon Tea's can be more than 1,500!

I ate scones and jam yesterday and it didn't hurt my cals as much as I thought it would!

It depends entirely on the afternoon tea and whether you want to eat it all! The last place I had it offered four small sandwiches, jam/clotted cream scones, mini chocolate eclairs, mini cheesecakes, a fondant fancy and a mini Eton Mess. I doubt I got any change from 1500 for that. That day I had brunch instead of breakfast, the afternoon tea, and a small-ish dinner (couldn't eat much because I was still quite stuffed after the tea), went for a long walk in the evening and just enjoyed myself. Think in terms of keeping a deficit over the week as a whole and you'll be fine.

Set aside some cals, do your Zumba, maybe eat something light before you go like a salad to take the edge off your hunger, and enjoy yourself

Spread out over a weeks time, one splurge meal means very little. Enjoy! And best wishes to tiur Mom!!!