Ownership of copyright should be covered in the agreement with the client. Your solicitor should be able to draft a contract in such a way as to give you rights under copyright law as well as under the contract. Such a contract may assist you in the event that a client uses your work before paying you. For example, it might provide that the client has no licence to use the work, or any assignment of copyright does not take effect, until payment has been received.
If your agreement does not expressly cover the issue, or there is no written contract, the owner of copyright will generally be the creator, unless there is an implied agreement to the contrary. The client, however, will have a licence to use the work for the agreed purposes and while you may be entitled to recover the money owing to you as a debt, you may not be able to stop the client using the work for the purposes for which it was created.