So you are desperate to go to England to visit family, we all are.
First off you need to take a Covid 19 test no more than 72 hours before you fly. One of our neighbours told me he had problems with this. He said that coordinating timing of flights and tests was tricky - you arrange one only to find you can't then arrange the other to fit in.
Before you can fly to the UK you also need to complete a passenger locator form for the UK government and you need to book and pay for your home testing for days two and eight of your 10 day quarantine.
When you arrive in the UK, border officers will be responsible for everything: checking your passports, checking your test result, your tests in the UK are booked and checking your passenger locator form. In some cases this has led to long delays and queues.
Since Spain is not currently on the 'red' list of countries, once you are in England you can quarantine with family and they are free to come and go from their home as they like. However, you cannot leave the house and have to stay put until 10 days have passed and you've got your day-eight PCR result back clean. Expect to be called regularly to check that you are abiding by the rules. You also have to record a video of yourself taking the PCR tests.
Having completed your stay, you then need to go and get a PCR tests for your return to Spain and of course you have to pay for all of these tests which will cost considerably more than the flights.
Restrictions in the UK about leaving mean that you will be taking advantage of the exception that you are a non UK resident who is ending a temporary visit. There is a form to fill in for this.
At check in, you will have to present this form along with your passport and TIE card. The rules say that you don't have to have a TIE card; the old green form/card or even proof of application for residency should suffice but there are reports of people being denied boarding without the new plastic card.
On your arrival back to Spain, you will pass through passport control where they will check your documents including the passenger locator form and a separate control where they will check your PCR test result.
The good news is that you are then clear to go, no need to quarantine in Spain and no further tests are required.
NB One positive test throughout the process changes all this and makes it infinitely more complicated. Also, if the situation changes and infections rise in either country, you will likely find yourself stuck in limbo with just a few hours notice.