Yeah I think I would go afternoon tea as well.
You could genuinely easily make enough scones for 30 - you'd need about 6lb of flour (1 decent sized fruit and 1 decent sized cheese each) and I think you'd have to do them in 3 batches but it shouldn't take more than a couple of hours.
Sandwiches, again either buy or cheap and easy to do yourself if you can make a friend/relative come over and sort them in the dining room. Waitrose etc will all sell suitable options.
Then I think I would want to do some nice extras - the place I went to did a tiny coronation chicken role, a baby Welsh rarebit and a tiny sausage role. You could again get the small rolls from Waitrose and the sausage rolls. Welsh rarebit super simple to at home or leave it out.
Cakes and patisserie again I think could be bought quite successfully.
Costing that up:
Scones will be £4-5 in ingredients plus clotted cream and jam (don't skimp on the jam or the cream) so call that £20 all in?
Sandwiches if bought would probably be £3-4 a head, so £140 but if you made them you'd get 3 finger sandwiches out of two slices of bread. Usually 14 slices in a loaf. So you'd need about 7 loaves of bread. Cucumber and cream cheese, smoked salmon, cheese and pickle and then either egg mayonnaise or a ham option would probably make sense. So the cucumber and cream cheese won't be more than £6-7? Cheese and pickle similar, and ham/egg. Smoked salmon probably £15-20 if you buy from Lidl/Aldi. So that's going to be £40 ish.
That then leaves £140 for things like buying the patisserie, tiny rolls etc. I think that would be eaten up fairly quickly buying catered food.
So then either ask family to bring a bottle of bubbly or alternatively a cake and use some of the budget on bubbly (depending what you think they'd prefer to do).
Ask local village halls/scout groups/WIs if you can borrow their giant teapots. Not sure where you'd get enough cups from, again possibly hire from local village hall? Some places do free glass hire for the bubbly if you buy wine from them.