I am sorry that you are disappointed, but US based airlines are not known for their lounges. I think that compared to AA for example, UA/CO offers ample amount of snacks. I enjoy the continental breakfast offering in the mornings very much too.

For example, in the DFW AAdmirals Club today, as usual during afternoon hours its just a dull snack mix and maybe some grocery store box cookies.

I am now in the JFK RCC and there are cheese & crackers, shortbread, snack mix, white choc covered raisins (I think lol), and some other kind of dessert snack mix.
Carrots and ranch.

Yeah, it's cheap - but that's what's come to be expected at United States Airlines lounges.

Hope you have a great trip to London.