1) See more of Slovakia and work our way back to Poland by train for a flight from Krakow (not booked yet and we need to work out feasibility/cost of fitting it in with our international flights).
To be honest, it's a bit weird to come back to Poland in order to fly to UK. Obviously there are plenty of things to see in Slovenia-Hungary-Slovakia, but perhaps it might make sense to look at other options (e.g. see below)
2) Take a train route that allows us to stop off a couple of times in places we haven't visited before or are happy to revisit - Salzburg, Munich, Cologne, Strasbourg seems to be on the main routes.
It's not a problem to travel (mostly) by land rather cheaply here, for instance:
Ljubljana to Salzburg: it's possible to get
discounted promo fares tickets from Villach (first big station next to Slovenia/Austria border) to Salzburg for only 9 euro, while leg from Ljubljana to Villach is just 12 euro no matter when you purchase ticket.
Salzburg to Munich: very easy and cheap, especially if traveling not alone - so called
Bavaria Ticket can be bought and is valid from Salzburg, plus it allows travel during the entire day, so for the same money it's possible to turn Salzburg-Munich into whole day excursion around Bavaria (e.g. leave Salzburg in the morning, arrive to Munich & leave your thing in hotel/luggage storage and spend rest of the for the trip to the mountains, lake Constance etc.)
Munich to Cologne: multiple options here: a) you can fly; b) travel by high-speed train (plenty of departures + discounted tickets, which can be bought from
German Railways website) and c) don't bother with in advance ticket purchase and simply use slower trains & Quer-durchs-Land-Ticket/Happy Weekend Ticket (more about these tickets on our Germany page -
http://europetrainsguide.com/Countries/Germany/Germany-DB.html )
Cologne to Strasbourg: again to avoid international tickets and in advance ticket purchase you can opt for Quer-durchs-Land-Ticket/Happy Weekend Ticket from Cologne to Kehl (which is small town just across the Rhein from Strasbourg). From Kehl to Strasbourg it's very easy by local transport
Strasbourg to London: again few option here: a) simply check flights from the nearby French and German airports or b) travel via Paris with high-speed TGV and then EuroStar trains (discounted tickets for both legs (from 25 euro & 51 euro respectively) available via
French Railways website 3) Wait to see what alternatives you come up with
Perhaps first thing I thought here was spending few days at the Adriatic cost (Croatian Istria, Koper, Trieste) and/or touring northern Italy (Udine, Venice, Verona, Milan etc.) with the idea afterward to fly to London from Pula/Rijeka or one of the Italian airports (all those are served by low-cost airlines, so it can be very cheap compared to by land travel option). In September it's still very warm there + less touristy, so it can be perfect ending for the European trip.