Upvote:5
There is a questionaire that you can follow from UK Border Control Agency to find out whether or not you need a visa.
Generally speaking a US citizen doesn't need a visa to enter the UK. But your concern is that you will be re-entering after a trip to Spain. Normally you shouldn't have any problems unless your duration of stay is getting closer to 6 months. Which in your case it doesn't plus given that you are only staying a couple of days on your way back you have "proof of onward travel" which normally is sufficient.