Benidorm must be one of the most expensive places to stay, so anywhere else will be better affordability-wise. Where to go depends very much of what your style of holiday is. If you like adventure, hiking and nature, the North West is amazing. Doing the Camino de Santiago (caminoways.com/) is mind-blowing. Hiking the mountains of the Picos de Europa is an experience you will not forget (chack this: online.wsj.com/news/articles/SB10001424052702303908804579566310546254896). If you have little ones, you can stay in a rural house in Asturias, Cantabria or the Basque country (check here: www.casasrurales.com/).
If you really like the sun and the beach, you can avoid the massively overpopulated beaches of Benidorm and drive up the coast 30-40km and will find absolute paradises for a fourth of the price, real Spanish food and atmosphere (that said, speaking a couple of words in Spanish would help). Valencia and many places in Catalonia are stunning and cost nothing... but as I said, keep away from "touristy" places.
If you like history, visit Toledo, Segovia, Salamanca, Sevilla or Granada, and you will be amazed by the architecture, ancient universities and the beautiful old part of town. Avoid restaurants where you can hear people speaking non-Spanish languages, because they will be the ones where the food is NOT Spanish and will be ridiculously overpriced. Dare to go into small bars and restaurants which look old and ragged, but will be serving truly Spanish food at a fraction of the price. Ask locals for the best place to eat, the best hikes to make and the best places to relax, instead of asking other British people. Adapt to Spanish times: have breakfast after 10 am, lunch at 2:30 pm and dinner at 9:30 pm and you'll have a much better chance to meet real locals and find truly Spanish restaurants. If it's open for dinner before 9 pm, it is not real Spanish.
If you like a bit of everything, rent a car and just drive around... get lost, explore small villages and talk to the locals. Try hard to show respect and interest for the language, respect the local culture, and you will be respected too. Show interest, and locals will show you the most amazing places, will invite you into their houses.