FFS, using real world countries and forces was only meant to act as proxies so we canget to the meat of discussing the tactics and strategies involved without having to spend the huge amount of time to come up with a purely fictional scenario for opfor to allow for a meaningful discussion. How likely that such a thing would happen in the real world is entirely irrelvant.
Also, if anyone bothered to think, they would realize that trying to defeat a force as vast as the PLA with PAP support inside of China with only light infantry would require the light infantry force to have numbers roughly equivilant to that of the Indian population to have much chance of success.
Light infantry needs massive numerical superiority to have any chance against a combined force army, the PLA and PAP, has both the firepower and would outnumber any other standing army in the world. That is one of the main reasons why there has never been a single case of anyone being able to wage an insurgency war against the PRC dispite said insurgents being trained and equipped and supported by the USA.
If you were truely looking for a scenario where a light infantry only force would have a chance against a combined arms conventional army, the PLA is just about the last foe you should be looking at.
If you want to country bash, please at least go do it in a thread that gives you a pretext to do so. If you really really cannot wrap your head around the idea of using real world countries as proxies, how about we call Scotland Kiltland and England FishnChipland?
Anyways, back on topic.
Firstly, I think 20k light infantry against 5k combined arms force is a little on the light side on both sides. There is just no way a force of 5k can hope to secure and garrion a territory the size of scotland. I think 20k combined force and 200-400k light infantry would be more realistic.
Even then, I think the light infantry would need more help, maybe we can flesh this out a little by saying that the scots had a referndum and overwhelmingly voted for independence, and the English said 'computer says no', which led to Scotland declearing independence unilaturally and the British sent the troops in. EU is royaly peeved and is actively supporting the insurgency with weapons, supplies and secure satillite communications, and enforcing a naval 'no sail zone' around scotland, but is not ready to get into a shooting war with the British, thus no direct military intervention.