I(personally) think ^^^^is different to moving a bun abroad. you would be taking it out of its comfort zone, and putting it through the most uncomfortable conditions, just so you can have your pets with you in Spain.
I really don't think that buns used to English weather would be able to cope with that of Spain, especially if your planning on taking them with you in August at the height of their Summer and when there is the biggest extreme (OK taking them from a English winter to and Australian Summer would be the Extreme), but can you see what I mean.
Unless you have a air conditioned room that they can live in permenantly then I would seriously consider giving my buns up, for what is best for them.
Please someone correct me! I know that the OP is attached to her buns