Hi - smarmier posters than I will doubtless be along any moment to gleefully correct you, but before the stampede, you should probably note that there is a subtle distinction between immigration and customs.

"Immigration" is passport control, and I think this is what you are asking about. As a UK citizen, you should not have to queue very long. As you will be coming through at a relatively slow time (i.e., not first thing in the morning when lots of transatlantic flights are arriving, nor on a Sunday evening when people are returning from weekends away), processing should be relatively quick. I would look for about 15-20 minutes, maximum.

"Customs" is the section, after you have cleared immigration, in which passengers are required to declare whether or not they are bringing in any item which is subject to restriction. There are three channels: passengers use the red channel and present themselves to customs officials if they have goods to declare. Passengers pass through the green channel if they are coming from outside of the EU and have no goods to declare (NB: passengers are occasionally randomly selected for screening anyway). Passengers pass through the blue channel if they have arrived from an EU destination (which you, coming from Spain, will be) and thus, in theory, they are subject neither to import restrictions nor occasional extra screening.

In other words, you should move through immigration relatively smoothly and should have no delays at all passing through customs.