Before advocating that our armed forces ignore LOAC "because our enemies don't observe it", perhaps we should make some effort to understand what LOAC actually requires.
This therefore implies, from a US Army perspective, that other nations (including the UK) who fly unmarked, armed MEDEVAC missions are not complying.
No. It is misleading to suggest the UK, by transporting wounded in helicopters which are armed and are not marked with the distinctive protective emblems, is "not complying" with the Geneva Conventions.
If that was not permitted it would be impossible to carry wounded or medical personnel on any transport without first removing all weapons and applying the red cross marking. Obviously that would be nonsense.
The point about distinctive markings such as the Red Cross is that they indicate an entitlement to protection. It would therefore be perfidy - a war crime - to use the Red Cross as a cover for hostile action. Without the Red Cross, you may lose the right to special protection, that's all. But there is no mandatory requirement to use the markings.
(There are special provisions in the Convention about "medical aircraft", but these are not intended for the hot contact medevac type of situation.)
Also, as another poster has pointed out, you do not lose the protection of the Red Cross if you bear and use arms for the protection of your patients and yourself.
It is certainly arguable that a MERT helicopter could carry weapons for self-defence while bearing the Red Cross marking. We are talking Chinook not Apache.
The UK decision not to use the emblem was no doubt based on practical as well as legal considerations. As 'NURSE' has said.