Reservations are mandatory in all high speed trains in/from/to France.

For Munich - London, you'll need at least a reservation for the Eurostar Brussels - London.

To France, you'll again need a reservation for the Eurostar. For the rest, it depends on where you're going in France and how much time you have to get back to Munich.

€*-as you most likely know, only does LOn- BRU=BE or PAR=FR. From/to there you can do without any REServ, but IN FR this will take much more time and stress=more changes, plus that SNCF is also strike-happy and seems to have started its strike-spring according to some records.

DE=Germany is fairly unique (but its also in UK that way) that it will let unlimited people in its fancy ICE trains and let you choose to do REServ or take the chance of having to stand-but then a REServ on those only cost 4€ extra.

In fact a BUSticket (flix or blabla) BRU/PAr-LON is often cheaper as even the Supplmt you have to pay for this €* But caution-the ferry they mostly use is now also under strike due to sacking of many of its workers (P&O ferries)

Or, dare I say again here-a cheap airticket MUC)memmingen)-LON(Stansted=close by to Cambr). Alas this is Ryanair-the very worst of airlines there is