I presume the point of a reciprocal taxation treaty is to capture all income and get taxed in one country (or they?ll eventually catch on).
From what I remember, 6,800 GBP threshold is for all annual earnings, and they?ll know this when you apply for tax credits in UK as you?ll need to report your Spanish income for that purpose. This is my logic anyways, I?m not actually British, but I worked in London and was surprised I didn?t get taxed on my first wage! hahaha That didn?t last long. Even if you go down the non-resident road, the other country will get your tax. There?s really no trick way out.
The 24% is the foreign non-resident rate, exactly. Think the only way to claim something back is via the UK path. If you?re on a resident rate in Spain, you?ll have deductions on taxable income such as if you?re single, married, rent, etc. So in your case, you?re not going to need a Gestoria (no tax returns need to be filed), but get yourself a good tax consultant in the UK!! Another rationale, from what I?ve overheard from my colleague who works in employment law, as long as you are taxed in one of the treaty countries, you?re fine. So if the UK deal works out better for you, ask the employer not to take tax out from Spain, and you organise to pay the UK tax man instead, as you would as a freelance. Or again, the tax credits are probably the same thing.
Anyways, I?m not really answering your question, but I hope it gives you an idea! Best get a tax expert if you?re really going to be collecting wages from multiple countries from your base country!! Good luck!!