Sure, here are the missing words filled in along with the explanations for each idiom:

15. You should be able to sell your stereo equipment for £200 - **at most** £250.
- **at most**: This idiom means the maximum amount or limit.

16. If you don't get out **at once** then I shall have you thrown out.
- **at once**: This idiom means immediately or without delay.

17. Pamela and David are always **at loggerheads**. I really can't imagine why they got married in the first place. They've got absolutely nothing in common.
- **at loggerheads**: This idiom means in strong disagreement or in conflict.

18. A chameleon is a remarkable creature - it is able to change the color of its skin **at will**.
- **at will**: This idiom means whenever one wants or desires.

19. You can't expect me to work overtime **at such short notice**! I need to be told at least two days in advance.
- **at short notice**: This idiom means with very little warning or time to prepare.

20. I was in a hurry for my train, so I chose a book **at random**.
- **at random**: This idiom means without a particular plan or pattern, by chance.