Hi David Torcal,
There is no tax relief for the taxes in Spain other than Spanish Capital Gains Tax.
In the UK you may be subject to capital gains when you dispose of the property.
You would need to obtain the market value for the property on the day you inherited it and convert this to sterling using an exchange rate in use at the time.
The exchange rate is not set in stone, which is why you have a choice.
Under the terms of Self Assessment, we do not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate for each acquisition and disposal.
For your convenience, you can use any of the rate available here:
Exchange rates from HMRC in CSV and XML format
Or from another just and reasonable source.
CG14200 advises how to computate and gain.
CG14200 - Computation: introduction
A list of incidental costs is shown here which can be deducted from the disposal value:
CG15250 - Expenditure: incidental costs of acquisition and disposal
Thank you.