Food wise I agree with your caterer. We are doing something similar to you, afternoon tea, followed by evening BBQ. We are doing full catering for each, as otherwise I think people will be hungry. Afternoon tea is no bigger than a normal lunch buffet when you think about it, and you wouldn't skip dinner and then drink a skinful of alcohol would you after only having some sandwiches and cake for your lunch?
I think, unfortunately you are going to have to do a full catering for the evening do, otherwise there is potential for annoyed and very drunk guests.
Drinks wise, for our afternoon tea we are providing tea, coffee and fresh orange only. If anyone wants alcohol with that they have to buy it. Could you do that instead? Its included in the £21 per head for ours and that way we get to have real champagne for our toasts.
Our catering goes like this:
Arrival drinks: Pimms or non-alcoholic fruit punch and posh lancashire crisps
Afternoon tea- savoury tartlets, sandwiches, cakes, scones, tea, coffee, fresh orange
Toast drinks- champagne or non-alcoholic elder flower cocktail
Evening do- BBQ for all 80 guests- steak, sausages, chicken, salad potatoes, coleslaw, greek salad, range of breads, there will also be enough salmon for 30 people and veggie options are stuffed peppers and vegetable kebabs. Wedding cake for afterwards.