If you have been left with nothing then I would say you have under catered. Was it £150 all in (meat, buns, desert, sides) or £150 on just meat? If all in then that is likely under catered.
You say it was butchers meat, how much did you actually get? If you were thinking 1 burger, 1 sausage, 1 chicken drumstick ect then you have under catered as that's not how everyone eats - especially if the bbq was cooking all afternoon. People will dip in and out and eat more when spread over the entire afternoon vs everything out at once or very close together. Or did you go to one of those butchers where they do x trays for x amount? Again for 10 adults plus the DC, depending on what trays you bought it would be under catered.
If you were planning on also feeding DC, why didn't you keep some bits back? So give the BBQer so much and keep back to be bbqd when DC got home.
I don't expect a lot of leftovers, leftovers usually means people were not entirely keen on the food. You might get some leftover sides (usually salad) but I've never hosted a bbq where there was lots of leftover meat - an odd couple of sausages/ burgers/ chicken wings or drumsticks.
With the drink, you should have filled ice bucket/ box and put all the guest drinks, plus what you were willing to share and softs/ mixers all on a table outside so people are not wandering the kitchen. Anything you don't want drunk you put away. People generally drink what is open, rather than open multiple bottles of wine. If your DH is topping up that's on him. It's very student vibe, drink only what you bring. If I host I usually cater and for drinks I will get a few bottles of wine (admittedly cheaper aldi wine that I also like), crate of beer/ ciders (depends what's on offer) and lots of softs and mixers. Fill big under bed box with ice and put it all out. I tell all my friends I have bought x, if you like you can bring what they like to drink - most will bring craft beer/ different brand of cider/ prosecco or something different. Sometimes it all gets drank, sometimes there's bits left.
Same with host gift, nice extra but certainly not expected. Amongst my friends we wouldn't do host gifts. Instead my mates all just muck in together and enjoy the company of each other.