Brussels IV was introduced in 2015 and applies to deaths after 17 August 2015. Although the UK have opted out of this clause it is still of relevance if you have property in the UK union and wish the Will you make in the UK to govern who inherits the asset.
The normal position is where you are habitually residing. For example, if you retired to Spain, and rent there, it is likely that you would be seen as residing in Spain. This may cause problems with the administration of your estate, however it may be queried if there is a jurisdiction to which you are manifestly more closely connected such as the UK. This new law is intended to simplify matters and a standard Brussels IV clause allows you to nominate when writing a Will the law of your nationality to apply to all assets in the Brussels IV zone.
It is always good advice to write a Will in the country the asset is held to avoid any doubt, however with the caveat of Brexit, this new law could be of practical significance for UK citizens with foreign assets.