It is always sensible to assert your copyright when submitting designs or artwork anywhere and for any reason, including applications for awards or commissions, pointing out that the designs or images are not to be used without your consent.

In the case of a commission or opportunity for which there is a lengthy application process involving a long development phase, you could enter into an appropriately worded Non-Disclosure Agreement with your potential buyer if this is appropriate.  This can be a simple letter signed by you and the organisation / company, which sets out respective rights and obligations.  It does not need to be in any particular form or worded in any particular way - in fact it is better to use straightforward and unambiguous language, which both parties understand.

If you are working with an organisation / company overseas buyer, it is advisable to include a ‘choice of jurisdiction’ clause - 'This agreement is governed by the Laws of England and Wales'.  Although copyright law is harmonized under various international treaties there are discrepancies with the protection offered under UK copyright law, which is why the inclusion of a UK jurisdiction clause in such an agreement is advisable.