I still think it is an 18. The evidence of what he said and his expressed attitude rules out that he was not minded to do some serious harm, plus the injuries are serious as well as suggesting more than just one punch. Quite possibly they resulted wholly or in part from being stamped on.
Offered a s 20 on a plea, I would go for the trial of the s18 on this, but have a separate alternative count of s20 for the jury's benefit. Then let the jury say which it is; I expect the CPS will do the same , given what you say. The best you could do, being friendly, is say to defence counsel 'If you plead to the 18, I'll open it low !"