The hotspot connection to your phone being active does not, necessarily, demonstrate that you have an active Cellular data connection available to iPad.
As you are evidently “roaming” outside of your home country (i.e., are in Spain, the UK being your country of residence - the latter being where your Cellular contract is presumed to exist with a UK Carrier) - with the demise of border free roaming within EU countries by UK-homed cellular devices - you might not have data roaming, or tethering whilst in Spain.
This contractual restriction, if present, would not be apparent from a technical standpoint; your iPad would still connect to the iPhone hotspot via WiFi/Bluetooth, but may not have onward connection via cellular.
As for “how”, with your iPad and iPhone being signed-in to iCloud with the same AppleID, your iPad will automagically (not a misspelling) be able to access your iPhone hotspot - unless you have chosen to disable Auto Join. From iPad settings:
Settings > Wi-Fi > Auto Join Hotspot > set to Never, Ask to Join or Automatic
If you change the setting to Never, your iPad will not automatically access your iPhone hotspot. Ask to Join will prompt before connecting on each occasion - where Automatic will, as might guess, automatically connect.