https://www.gov.uk/marriage-abroad/y/spain/uk ; (New Rules)

http://www.hitched.co.uk/chat/forums/t/156627.aspx

They will need a document from the British Embassy staing they are free to marry ie not currently married also their birth certs will have to be translated into Spanish.

Likely documentation you will need:

    Long form birth certificate – notarized and translated into Spanish.
    Valid passport and at least four copies – to be used when you apply for the Empadronmiento, Residencia and file for the marriage.
    Certificate of Marital Status (Certificado de Estado Civil)
    Certificate of No Impediment (Certificado de No Impedimento), which can be obtained from the registrar office of the home country.
    Divorce degrees (Certificados de divorcio) – if applicable
    Spanish Town Hall Registration Certificate (Certificado de Empadronamiento or “Padrón”)
    Extranieria application form – print three copies. Complete the extranjeria form and file it at the national police station. Remember to bring with your passport and a photocopy of it.


Empadronamiento
This is the process of registering yourself as a resident in your province. This is required in some provinces but may not be applicable everywhere. It is best to check beforehand. Fill out the form (it’s the same as the extranjeria form) and take your passport and a photocopy of it along with you. As you are registering yourself as a resident of your province you will need to have an address. This can be a rented address and you will need to show your rental contract.

Step 2: Submitting the paperwork

Bring along a photocopy of your passport along with the necessary documents when you file for your marriage at the local registry office. You will also need one of your two witnesses to accompany you and to bring his/ her passport. The officials will then process your information and register you on their system. The intent to marry is then displayed on the public notice board in their office for 21 days. After 10 days, you can call the office to request an interview date - part of the marriage process.

If you do not speak Spanish it is advisable to either hire a Spanish solicitor or to get someone who speaks Spanish to help you with all of the paperwork so that the process runs smoothly. Also, registry offices can be quite busy and it is usually better to go earlier rather than later in the day.

Step 3: Waiting and going for the interview

Once the 10 days are up, you can call to arrange an appointment for the interview process. Be advised that you may have to wait up to six weeks for the appointment date. At the interview you may be interviewed separately and asked to answer a number of questions to make sure that you are genuine applicants.

Ceremony

A civil marriage can be held in the courts or the Town Hall of residence, performed by the Mayor or a designated councilor. The marriage is effective immediately following the ceremony. After the wedding, it will be registered in the Civil Registry and a certificate stating the date, time and place of the marriage will be issued.

A religious ceremony (or blessing) can be held following the civil ceremony if desired. You should be aware that after a religious ceremony, you have one week to present the church-issued certificate to the nearest civil registry. If you fail to register the marriage will not be recognised.
http://www.expatica.com/es/family/Partners/Getting-Married-in-Spain_13864.html