'I know there are kits out there for lots of money, but I am happy with just a
M-Lok handguard plus the shorter outer barrel to go with it.'
There are a variety of clone rails of both the BCM MCMR and various Geissele rails out there. Ultimately depends on how much you want you outer barrel protruding out of the hand guard, once you've picked your ideal length then you'll have a better idea of what's available to you.
Angry Gun MCMR clones come in 8", 10" and 13" if memory serves correctly. The clone Geissele rails run from 7" up to 13.5" (I think) from a whole host of brands in a variety of flavours. HAO seem to be regarded as having the best MK16 (9.3" and 13.5") rails out there for a decent price. Though I don't believe HAO are currently shipping to the UK due to COVID. I'm sure there are plenty of others, theese are largely just what are 'hot' right now.
No idea on outer barrels for the MWS specifcally I'm afraid, and depending on which rail you go for as mentioned by
@AlphaBearyou'll possibly need an adapter to get a rail made for AEGs to fit it (the adapters are essentially just a shim which keep everything locked in place).