You could fly from Bristol to Alicante tomorrow at 0800hrs with EasyJet. Flights only will cost you £145.98p each or you could book a package holiday with them which, on a half board basis, would be from £287.37p each.
http://www.easyjet.com/asp/en/book/index.asp?lang=EN
Alternatively, if you can wait until Sunday, the 15th, then you could have a flight from Cardiff to Malaga, with Thomson Airways, for £93.98p each.
http://flights.thomson.co.uk/en/index.html
The flight only prices above don't include luggage charges.
Or there is a ferry from Portsmouth to Santander, leaving on Thursday, the 12th. To return on Saturday, the 21st, the total cost would be from £514.00p. This is with Brittany Ferries.
http://www.brittany-ferries.co.uk/