I have some humbrol metalcote which I find excellent for reproducing steel finishes, however I had issues when thinning it. Adding normal Humbrol enamel thinners started to turn it into a gluggy mess. I used laqcuer thinner and it's now the perfect consistency, however it starts to dry almost instantly, and is a pain to apply as it goes on and smears straight off unless great care is taken which can leave it a bit thick.

Are there any additives I can use for enamel/lacquer paints to slow down drying time?

Mate i dont know there on the west coast but if it's hot like here you can try to wear a jumper,then raise the aircon on maximum maybe you slow down enough the drying time,i dont know about additive to slow down laquer or enamel only the opposite

It's already pretty cold here at the moment so no joy in that regard. I fear I may have overdone the lacquer thinner which is causing it to dry so quickly.

Which thinner did you use?
The guggy mess was caused by the wrong type of thinner and the other thinner you used sorted (solved) the guggy mess but it dries too fast.
I haven't used the metalizer paint but if I had this problem I would try to use balsam turpentine.
/ Robin