1) The Synonym Finders 1- 8 in COAsT (English – archived tests)
2) Letts Vocabulary Results Booster (given away free at some of the Chuckra Mock Tests)but can also be purchased.
3) Synonyms and Antonyms by Christine Draper.
4) Chuckra Vocabulary Cards
5) CGP KS2 English Targeted Question Book: Spelling – Year 6
6) Read these items for guidance: Improve Your Vocabulary, Parts 1, 2 & 3
I hope that helps.