I’m sorry to hear of your loss, but as it’s Easter week-end, it may be some time before a specialist will reply, and there are some obvious issues to consider.
Did you own the house jointly, and is this clear on the title deeds ?
Did your husband have a will ? If the answer is yes, did he leave his share of the house to you ?
If there was a will, did it specify applicable law, Spanish or, presumably, U.K. ?
Do you have any children who could have a claim on your late husband’s estate ?
Unfortunately, you have been forced into one of the most difficult situations a widow has to face, and, you will certainly need the advice of a Notario. However, I suspect that the bottom line is that you would not be able to sell the property until all of the legal issues have been sorted out.