You could try calling again and see if you get a different agent and they may be able to take the payment from you, which would hopefully then allow you to OLCI.

If you have to do it at LGW, payment is taken at the Customer Service desk in Zone D. Go there first as a check in agent would need to refer you there if you got to the desk first.

BAH - I know what a lounge dragon is, but "spa serpent" is a new one on me. I can hazard a guess at the answer but perhaps you could confirm if this is what I think.
PS - We don't have an MB Spa at LGW so don't recall seeing any serpents!!