If you didn't have a permanent home in either state between the date you sold the UK property and when you began to reside in Spain, then it seems that assuming you are a national of only the UK, there is nothing tying you to Spain until you began to reside there. So assuming you remained UK tax resident under the SRT up until the point you began to reside in Spain, you would remain treaty UK resident up until that date.
Not sure I agree with the above.
In examining whether dual residence arises it is necessary to breakdown the analysis for the period 1.1.19 to 5.4.19 and 6.4.19 to 31.12.19.
Re 6.4.19 to 31.12.19 IF you were UK resident for this period (ie would need to be UK resident for the whole tax year 19/20) and also Spanish resident only then the dual residence require to be resolved.