Very sorry to hear your news. I had cancer in 1997 but I was lucky in that it was operable. I hope your partner is lucky too.
The advice by Steone is good.
If you partner is able to travel to Spain it is very straightforward to make a Will in Spain. If there are no complications in the allocation of assets there is no reason to employ a lawyer. Whether you do or not the Will can only be signed in front of a notary who is the one who draws it up. Thus they have more experience in doing so that any lawyer. I and family members have made a number of Wills in Spain and never needed a lawyer. (I am a former UK Met Police Company Fraud Dept detective not a bar room lawyer).
When you say ‘partner’ the law varies on inheritance according to the exact relationship. A couple living together, with no paperwork to make that relationship recognised in law, I believe you are not regarded as anything but friends and the tax free allowance was, and I believe still is, close to only 17,000€, whereas for married or legally recognised partners it is much more generous. With the average holiday proper that would probably mean no tax to pay. I own 100 % our apartment which at is least averagely priced and my wife will not have to pay any inheritance tax . But as Steone says, take advice.
Not having a Will in UK or in Spain will always lead to expensive complications and involve courts and lawyers.
Also be aware, when one makes a Will in UK it starts with ‘This is the last Will and testament’. If you then make another Will in Spain, you may need to add a codicil to the UK Will to say it is has not been superseded by the later (the last) Spanish Will