Leaving UK and working remote for UK company whilst in transition of receiving residency

I've recently moved to Spain. My company has agreed to this move as we have a Spanish presence and I will simply transfer contracts to our subsidiary. I'm a UK citizen with a EU citizen spouse. I'm in the process of getting my family residence permit so there's no issue around my ability to live and work in Spain. My concern is that the current backlog with Spanish immigration authorities means this process will take longer than 6 months. I've been told by my Spanish advisers that Spain do not consider me a resident until this process is completed and it's considered that I'm working remote for my UK employers. I'm now unsure of the ramifications of this given that from UK perspective I may not be considered a tax resident and whether I can still be employed by the UK company? Other than the P85 form, is there anything else that would be required once I reach the 183 day threshold?

Hi,
If you are in Spain and working there for a UK based employer, you are taxable on your earnings in Spain.  
All HMRC would require is the P85.
Thank you.