Most Primary Schools take part in the Primary Schools Maths Challenge each year, where the top performers get given bronze, silver or gold. In each school the gold awards then go forward to be tested against other schools nationally. Your maths co-ordinator should have details.
The primary maths challenge is excellent, but just to clarify: the school decides who gets the bronze, silver and gold awards, there is no national standard. I used to give the top 2 scores gold. Some years that may have been a high enough score to get to the final, but other years it would be no where near.
Our county also has a primary maths quiz, with preliminary rounds with the winners of each heat going to a county final. In our county the challenge, which is excellent, stresses problem solving, mental arithmetic and relies on excellent group work.