For the London gardens one is there a missing part of the clue which would give the 'Nick' in Nick Kershaw
The two you have left remaining are:
1) Able to vote, fish or onion
2) This one flows, thread or crest
I don't know the answers but these thoughts may help boxtops, grasscarp etc get to the answer.
1) Able to vote, fish or onion.
-Able to vote could be adult, man, 18, suffrage, elector
-fish could be a type of fish or could be a synonym such as net, bait, hook, angle
- for onion I can only think of pickle or spring
2) This one flows, thread or crest
- This one flows could be a river (Nile, Po, Dee. Tyne...)
- thread: cotton, string, wool,...
- crest: peak, head, top