yep it's true, all I can say is if you have moved to Ibiza in the same year as selling your Uk home then you are potentially in the sh*t, or if you have moved to Ibiza and have you Uk home up for sale , you are equally in the s*it. Its ok if you havent made any dosh on your home in the Uk but if you have then you have a problem. This year I got my son into school in Ibiza (he loved it) with my daughter following, we were very committed to moving full time until we discovered the rules and have now delayed. Now we are back in the uk until we are able to move without the potential of Hacienda wanting to collar us.
I am only telling my own side as I am a brickie, not an accountant so you should make your own investigations. The double tax agreement between spain and the Uk is available to read online from HMRC.com and you should concentrate on the capital gains section and the tie breaker rules for residency, those are the important bits. Biggest problem with the tie breaker rules (there are 4) is the center of vital interest one, it isn't definitive as to where your centre of vital interest is, so if you get that far down the list of tie breaker rules you could be in trouble.
Not sure of the rate of tax but it must start at around what you said, could be higher.