Hi, you can check if there are seats left via b-europe:

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Keep in mind, that they do not write how much seats that are left for Pass holder.

There is no 3 days pass, the minimum is a 4 days in 1 month Global Pass. 

If you are doing only this trip you could check if it cost less to book one way tickets. If you book via Deutsche Bahn using Eurostar services to Brussel and from there ICE services to Frankfurt with connection to Munich, you can use the DE Connect fare, which cost less than one way/return ticket Eurostar + ticket of DB.

There’s a direct TGV Paris-Munich. So travelling via Paris seems a good option well.

I checked prices via DB in about a week, and it gives 168 EUR (one way). The cheapest global pass option with surplus reservations will be quite similar I think for the options DB Navigator gives.

I admit to being confused:

A 4 day pass is 246 euro. Eurostar supplement 30 euro each way, D Bahn ICE trains no res needed. UK connections reservation free, so total return cost just over 300 euro.

Cheapest buy ahead Eurostar from 49 to 149 depending on date. So total return say 150 euro. Brussels to Munich return is showing 200 to 250 euro return (d Bahn).

UK to London may be another 200.

Total approaching 600 euro with little flexibility or refunds on saver tickets.

Just a small point - you would need a Global pass which has a 4 day minimum.

Also I assume you will be travelling to St Pancras and you know you can use unlimited trains in the UK on that travel day.

@Yorkie ,did you check booking the whole lot at Deutsche Bahn? London-Munich can be done for +/-  300 euro with ordinary return ticket next week. But I think cheaper is possible, if you book well in advance. 

It's a wonderful tip of @Angelo, much cheaper than separate booking. Didn't know that!

PS you can use the Germany pass (with three days) from Brussels on the ICE. Combining that pass with an ordinary Eurostar saver return, could be cheaper as well. 

D bahn is showing the prices I quoted from Brussels to Munich, Eurostar for the prices booking with them.

I do not dispute that there are convoluted ways of presenting prices, each of which has their own terms and conditions. I still think the combined cost of a fully flexible Global pass with relatively cheap reservations if plans change is the best way for a Brit to travel on this itinerary.

Savings by other routings, even if they can be realised for the specific days and times the poster wants, are tiny and almost  certainly inflexible. There is also the opportunity to use the pass to break your journey and/or extra travel in the UK to and from St Pancras.

There is a special agreement between Eurostar and Deutsche Bahn, if you book via DB International platform Tickets London-Germany with change on ICE services in Brussel you pay a special lower price for Eurostar services (DE Connect fare). To get to pay less, it also needs to find an ICE that is not full to get discounted Super Sparpreis Tickets. Also it could cost less a return ticket of Eurostar. DB has no return tickets, but only one way. 

All other ticket via DB International are simply DB Tickets + Ticket of the other train company, same thing like Trainline and Co.

If booking is possible via DB (bahn.de) you get one ticket for the hole trip. (Possible from Stockholm to Bologna, or Paris to Budapest)

Here an article about this:

https://jonworth.eu/how-to-use-deutsche-bahns-de-connect-tickets-for-train-trips-between-germany-and-london/ 

A worked example for Mon 14/11 shows the DE connect price for the 1104 E* to Brussels then to Munich by 2110 would cost 268 euro. Wednesday it drops to just over 200 euro. (And yes the E* ticket shows de-connect).

Eurostar Interrail availability that week is generally good with plenty of choice for earlier trains which DB is not showing.

So best return fare for only a small choice of trains is still well over the 246 + 60 a Global 4 day pass would cost.

I accept that highly numerate, flexible travellers with loads of time to browse D-bahn and E* could possibly find a combined fare on some days for less than 300 euro return, but it is a much simpler activity to check E* on b-europe for a decent train, find connections on d-bahn and still have 2 days left on my pass.

..or you could just present the options and let the person travelling make the decision which tickets they wish to purchase.

In no way was I suggesting anything other than that. In fact I thought I was merely defending that very thing.

I was though trying to show that the whole premise of an Interrail Global Pass is one of flexibility and value for money, and more importantly requires very little additional budgeting or research beyond planning your trip.

(if you use a guideline of 35 - 55 euro per travel day plus reservations that is the total cost of your travels, irrespective of which trains you choose. Obviously the minimum travel days is 4.)

Once you know how many travel days and choose your pass the total is easily calculated.)

I have no issue with those who want to spend many happy hours planning and investigating the many options of pre-paid prices, especially for journeys across Eastern Europe. 

In fact until recently I often spent many a happy hour planning independent holidays with the cheapest flights. That involved several low cost flights, several airports, flexible dates and durations and even a choice of arrival points.

really appreciate all of the input its really insightful and I am somewhat staggered as to how much knowledge there is and how many options there are!  I've learnt a lot today - thank you everyone