I have held full uk driving license for 30 years and am in the process of exchanging it for a Spanish one. Given that almost all car rental companies require you to have held your license for at least a year, when I receive my new Spanish license can I use it to hire a car in the uk immediately or will I have to wait a year?
I exchanged my British driving licence for a Spanish one in 2012. Unlike the British licence, the Spanish one doesn't have a counterpart: all the information on validity is on the plastic card. This includes a table on the back showing your entitlements. Mine shows class B valid from the date I passed my test in the UK (in 2003). It also has an "Observations" field with a code which says that my original licence was from the UK.
Note that you may lose special entitlements. My UK licence had a ton of special classes, but my Spanish one only has class B. Still, that's what matters for hiring a car.
As per this article from an automobile magazine, you will probably get a license with an special code with key 106.3 with the first issuance date, so you should be able to provide proof of experience.
I'm not sure if this code is a national code or a EU-wide code, though.
As of right now (pre-Brexit) you can use your EU driving license in the United Kingdom. There is no need for you to wait for anything.
If and when Brexit happens it is unclear what the policy will be.