score:3
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.