The tax will vary according to the city you buy the guitar in, and the exact price; here is some interesting information re: the 'IVA' from http://www.tripadvisor.com/Travel-g187514-c104194/Madrid:Spain:Money.Saving.Tips.For.Spain.html
: "The Sales Tax Refund - There is a sales tax, called IVA in Spain, which is a value added tax of 16% on retail sales. You can get a refund on this if your purchases are at least 90 euros at one store. Look for stores with the Tax Free signs. They will give you an invoice which includes the price and the tax on each item. You can accumulate these invoices if you shop at different stores. When you go to the airport to depart, look for the Spanish customs desk. You must have your invoices and the form asking for the refund filled out. You then show the inspectors the items you have bought and the inspectors will stamp your form. After going through the airport inspection, look for a mail box where you can drop the form. Your credit card will be debited in about 2 months with the IVA refund."
You should declare it when you re-enter the UK, if the Customs Declaration form says you must declare items over a certain monetary amount, and also if musical instruments are listed as a declarable item. Before you leave, you can inquire from your local customs office, what the rules are re: declarations, so you're prepared. Because the amount of customs tax is dependent upon the price/value of the item, it has been suggested to ask the store where you buy it, to write you a receipt for an amount that's less than what you actually paid, to try to save money, if you should choose to do this.
Have a great time, I hope your husband enjoys the guitar, and congratulations on your recent nuptials!