Can I self-isolate in a different country from the UK and then travel to the UK without being in solitary confinement again?

I am a Spanish citizen and I would like to travel to the UK from Spain, Edinburgh more specifically. But currently, I would need to self-isolate for 14 days since the country is not on the list of exemptions since the 26 July.

Spain (this does not apply to arrivals from 00:01 on 26 July 2020)

I have friends in other countries which are exempt to make the confinement, Poland for instance. Then I could spend all those days in their home and avoid expensive accommodation in Edinburgh, I think it is not worthwhile paying so much just to stay in a flat all the time. So, my question is:

Can go there to spend the 14 days in self-isolation and then justify in some way that I have been there doing my confinement. Is this possible?

Actually I don’t know where I could ask this question, is there any contact number or email from the UK government where I can reassure this?

I also found that some people could not travel from the UK to Spain by plane? Can that happen to me if the rules change these days?