When I retire, will I be eligible for a state pension from the UK even if I am then in Spain? [...]
[...] And would that cumulate to the one I will get from Spain?
[...] I guess my personal pension scheme won't be a problem and I should be entitled to get the all money out as I want based on UK rules even if ...
There is an interesting read on Spanish inheritance tax at https://www.spanishpropertyinsight.com/2016/02/08/spanish-inheritance-tax/
Unfortunately, it describes this situation (UK estate, Spanish tax-resident inheritors) as "a scenario you categorically want to avoid for your heirs at all costs. It entails for your loved ones spending greater time, money ...
After 10 working/qualifying years in the UK you will receive an UK pension as things currently stand. The amount will increase to the full amount as you accumulate more qualifying years.
If you take it abroad, it may not be index-regulated, however, which seems rather unfair