"RAT IN THE KITCHEN," UB40. A&M.; On its previous albums this English band's social-political reggae tended to sound formulaic and dull. But with the quintet's latest and best album, it has finally established its own engaging identity. The songwriting is sharper and more melodic than in the past, and the arrangements and tempos are more varied and uplifting. No longer strict reggae traditionalists, this newly inspired octet isn't averse to showing some ska-like snap with such energizing tunes as "All I Want to Do." The song "Rats in Me Kitchen" even features a lengthy trumpet solo by Herb Alpert. When the quintet launches into an anti-apartheid or pro-proletarian tune, it now has the musical muscle to give these admirable sentiments the necessary conviction and bite.