Passport control, UK: Probably no problems. (They tend to ask questions about unaccompanied minors leaving the country but they're mainly bothered about young people running away from home, or similar situations. They probably won't give you too many problems leaving for a group holiday).
Passport/Immigration control, Spain: Unlike Portugal (where you would find it hard to get in without an adult) you probably wouldn't have any problems.
Tour operators: No tour operator is likely accept a booking from anyone under 18 years old. (You're too young to enter into a legally binding contract). This means that you're almost certainly going to have to try to arrange your accommodation and flights separately . . .
Flights: Many airlines (e.g. Ryanair) will only accept 15-year-olds on their flights if a parent (or guardian)sees them off and remains at the airport until take-off. (It's not clear from Ryanair's site how you can board a return flight when your parents are still in the UK and so can't see you board the flight back!). Some airlines will only accept bookings made using the credit/debit card of the 'lead passenger'. This might give you problems making the bookings.
Accommodation: You probably won't be able to book accommodation through a UK agent, for the same reason that the UK tour operators won't take your bookings. You'll probably have to find some way of booking direct with the hotel or through an internet booking site which will, say, accept one of your parent's credit cards, even though they're not travelling. Even if you can find a web site which lets you book, it would be best to speak directly to the hotel to see if they'll let you stay there.
What you're proposing is probably not impossible but it will certainly be very difficult to arrange.
Chris