For future updates, pleas speak with your accountant. Below is an explanation on how the rates are worked out:
For tax year 2121 so Renta declaration 2022
Reference rate Friday 31/12/2021 0.84028 or €1.19007 per £ sterling
Average rate Friday 31/12/2021 0.85960 or €1.16333 per £ sterling
Rate for Modelo 720 €1.19007 per £ sterling
For tax year 2020, so Renta declaration 2021
Reference date is Thursday 31st December. Reference rate was £0.89903 per £1 or €1.11230
Average rate for 2020 was £0.88970 or €1.123970
Rate for Modelo 720 is €1.11230 per £1 sterling
For tax year 2019, so RENTA declaration 2020
Reference date is Tuesday 31st December. Reference rate for the day was £0.85080 per €1 or €1.1753 per £1.sterling
Average rate for 2019 was £0.87777 or €1.13925 per £ sterling
Rate for Modelo 720 is €1.1753 per £1
For tax year 2018, so RENTA declaration 2019
Reference date is Monday 31st December. Reference rate for the day was £0.89453 per €1 or €1.1179 per £1.
Average rate for 2018 was £0.88471 or €1.1303 per £1.
Rate for Modelo 720 is €1.1179 per £1.
For taxyear 2017 so RENTA declaration 2018
Reference date is 29th December as the 31st was a Sunday. Reference rate for the day was £0.88723 per €1 or €1.1271 per £1. Average rate for 2017 was £0.87667 per €1 or €1.1407 per £1.
The rate to use for Modelo 720 is €1.1271 per £1
For taxyear 2016, so RENTA declaration 2017
Reference date is 30th December as the 31st was a Saturday. Reference rate for the day was £0.85618 per €1 or €1.1680 per £1. Average rate for 2016 was £0.81948 per €1 or €1.2203 per £1.
The rate to use for Modelo 720 is €1.1680 per £1.
For taxyear 2015, so RENTA declaration 2016
The exchange rate on the 31st December 2015 (British pounds to Euros) was €1.3625. The average rate for 2015 was €1.3777.
There is nothing in the tax legislation with regard to the exchange rate to be used in the income tax declaration, only the Modelo 720 ( €1.3625). If you receive euros already converted (e.g. state pension) then you could work out the rate you received, but it won’t be far out from the average rate, as this is based on the daily average throughout the year. Most people use the average rate if they receive sterling funds and then convert as and when required, although technically if you make an exchange profit then this is declarable.
Quite a lot of gestors use the average rate, but others use a different rate as evidenced above. I suspect that they use the same rate for all their clients, as this is the easiest for them.