Buses in Spain are comfortable and efficient. Although described as 'buse services', the vehicles plying the routes are usually an air-conditioned coaches. To the tourist the Spanish bus system may seem a bit complex especially as there are many lines covering long distance journeys. However buses are recommended for both their comfort, price and convenience for short trips and for longer excursions.
If you know more or less the journey you want to make then and just want to buy your ticket try the ALSA webite seems to have it all.
If you want know how to go to Spain overland from UK then National Express has a service which links most of the main cities in UK to Spain.
Links To Bus Companies
If you want to examine some of the options available to you in more detail follow some of the following links.