You should be able to go to your local library and check England and Wales deaths on www.ancestry.ac.uk up to the end of 2005.
If that turns up nothing then you will need to send a search request to www.gro.gov.uk requesting they search from Jan 2006 to date - there will be a fee. You can give them added information like her date of birth, to distinguish her from other people of the same name.
As a UK citizen living in Spain, if she has died, this probably should have been notified to the British Consul - I haven't time to go trawling through the website, but it would be worth ringing the GRO and asking how you would go about searching those records.
I hope you are able to get the answer you are looking for.
You can check the indexes for births deaths and marriages up until 2005 on ancestry, findmypast or in some larger libraries they have copies of the GRO/st catherines house indexes.
You can't check death certificates online anywhere.
If you think that she may have died in the Uk and have an area where you think she may have died - then could you contact the local crematoruims, there may be of course more than one, let them know that you are searching for a burial location of her ashes and want to know whether they conducted her funeral/cremation or burial.
As she was Spanish she may have a more unusual name which would make it far easier to find and a lot of crematoriums are used to loved ones searching for burial/crem information from estranged family.
How sad. I hope she is ok. If she has died and it was before 1994, it looks like you could check the information online for free. www.uk-family-history.com/overseas-BMD-records.html However, it sounds as if it is more recent, so you may have to ring the GRO. Perhaps it might be easier to write to the address she was last at. Good luck.
We think something has happened to her because DSD has rung her in Spain loads of times and had no reply, she has contacted DH's sister and her cousin and had no reply. DSD was always friendly with everyone and not part of the original falling out.
DH has emailed and facebooked several relatives asking about his mother and none have replied.
I found my DH's mum's death certificate on one of the geneology websites, ancestry.com I think. They have death certificates up until quite recently, and you can check them online, but I think they only cover the UK so wouldn't help if she died in Spain. Sorry for your DH's loss.
In the UK and if no other member of the family can give him an information, suggest to him that he writes to the registry of births deaths and marriages in your town/city giving as many details about his mother as he can. Not sure how he would go about it if she died in Spain.