You can travel, as Spain is currently on the amber list, but you are treated as not fully vaccinated for the purposes of (eg) quarantine-on-arrival and post-arrival testing. The quote from HMG's website is not, it seems to me, in any way ambgiuous: being about people who have natural immunity though prior infection and a single dose of Moderna, it refers to your exact circumstances.

You can travel, as Spain is currently on the amber list, but you are treated as "not fully vaccinated" for the purposes of (eg) quarantine-on-arrival and post-arrival testing. The quote from HMG's website is not, it seems to me, in any way ambgiuous: being about people who have natural immunity though prior infection boosted by a single dose of Moderna, it refers to your exact circumstances (unless I misunderstand).

I accept that Spain considers you to have been fully vaccinated, but I note that none of HMG's regulations delegates to a foreign agency the right to determine whether someone's been fully vaccinated: the definition of full vaccination is set out in the regulations. It only allows certain foreign agencies to certify that a person has had the medications required to satisfy HMG's definition of full vaccination.

You can travel, as Spain is currently on the amber list, but you are treated as not fully vaccinated for the purposes of (eg) quarantine-on-arrival and post-arrival testing. From HMG's website, which specifically addresses the issue of someone who's had COVID-19 and a single Moderna dose, which if I understand correctly is your situation:

Natural immunity

Some people may have natural immunity, for example if they’ve had COVID-19.

Some countries relax their entry rules if you have natural immunity. England does not.

Follow the rules for those who are not fully vaccinated if you have natural immunity and haven’t had a full course of an approved vaccine. For example, if you’ve only had 1 dose of Moderna instead of 2.

You can travel, as Spain is currently on the amber list, but you are treated as not fully vaccinated for the purposes of (eg) quarantine-on-arrival and post-arrival testing. The quote from HMG's website is not, it seems to me, in any way ambgiuous: being about people who have natural immunity though prior infection and a single dose of Moderna, it refers to your exact circumstances.