Corona virus test 48 hours before departure

Good question, I imagine only be from the result, with the rapid test pretty well instant. Many visitors from UK now bring their own kit and you snap the result and load to the web site.  

Interestingly, for multi-leg journeys, there's an option to take test within 2 days of departure on your first leg, so you could take the test before setting off in Spain, then no worry if you take more than 2 days to get to UK

 Gov Travel Advice website

Taking a test on your journey to England

If your journey to England is long, and will involve stopping in another country on the way, you should try to take a test in the country you’re travelling through. This is so you take the test in the 2 days before you board the final service to England 

For example, if you’re driving from Germany and you stop for a few days in Belgium, you should take a test in Belgium.

If your journey to England is a multi-leg journey, you can take the test in the 2 days before the start of the first leg. However, where possible, you should get a test within 2 days of your final departure point to England.

 "there's an option to take test within 2 days of departure on your first leg, so you could take the test before setting off in Spain, then no worry if you take more than 2 days to get to UK"

It does seem to say this but at the same time also states "However, where possible, you should get a test within 2 days of your final depart"

My guess something they might argue at the border, I presume the ports have a rapid test facility.