Neither British nor French can be used as a singular noun. For example, (a) is ungrammatical.
a. *A French/British is dancing.
Although the French/British can mean 'French/British people' collectively, French/British is usually analyzed as adjectives. For example, (b) is grammatical and natural English.
b. The French/British are struggling.
Can British and French be used as plural nouns, meaning 'British people' and 'French people', respectively, outside the construction The French/British?
For example, is (c) grammatical and natural? How about (d)?
c. A lot of French/British are dancing.
d. Several French/British are dancing.