"Tea with biscuits" is a single noun phrase representing the entire food preparation, and so the verb should be "is" to agree with the singular subject. This makes sense given that you say "a very nice combination".
However, using a different phrase for the second half of the sentence gives us some choice:
Tea and biscuits tastes delicious.
Tea and biscuits taste delicious.
In the first case, our use of "tastes" implies that "tea and biscuits" is treated as a single noun phrase representing one food preparation. This sentence makes a particular comment on the flavor of tea and biscuits together. Alternatively, we could treat "tea" as one part of a compound subject and "biscuits" as the other, which requires our use "make" in the second case. This sentence implies that both tea and biscuits taste delicious individually. Both of these sentence are grammatically correct as long as the verb choice matches our intention.
Tea with biscuits are a very nice combination.
This sentence is incorrect. "Tea with biscuits" is not a compound subject, because "with" is a preposition and not a conjunction, so the verb must match a singular subject.