London - Lille - Lyon - Milan?

How about via Croatia, including a scenic trip along the Sava River. You can get a ferry from Split or Dubrovnik to Bari. (Although, since you're continuing to Greece, you might be able to avoid Bari, and sail directly to Greece.)

There are a million places to stop. I would likely pick somewhere small but doable in Austria or Switzerland. Berne is beautiful and walkable. Or Kufstein near Innsbruck (I had an hour there between trains, and was stunned by how beautiful the area was)

Hi 

Thanks for your input.

Croatia sounds good but without crossing the Adriatic twice, it’s difficult to get down to Greece from there.

London - Bern (overnight stop) - Milan for overnight train to Bari is sounding good to me although possibly replacing Strasbourg for Bern would work well too.

The attractions to me of both the above are that I don’t have to go through Cologne or Paris Gare de Lyon (only because I’ve been to both enough times) and my route down to Milan is via Brig which is a line I’ve not taken.

I’ll research what the cost might be for 2 people on the Italian sleeper for our own berth..

Yes, the ICE or TGV to Strasbourg saves a lot in reservation fees vs the direct TGV-Lyria to Switzerland. Only 10 min walk from Gare du Nord to Gare de l’Est, too.

Then either stopover in Strasbourg or TER + Swiss trains to Bern. You could also try to work in a trip over the Bernina pass if you have time.

Passholder reservations on the Italian sleepers are quite costly. You might find a regular ticket is cheaper without the pass.

That’s the £51 price per person for sharing  a 2 bed cabin which doesn’t sound too bad. It’s £122 per person without the interrail.

That sounds decent.

I was offered a €122 with Interrail in a private cabin vs €90 for buying it outright!

Yes, especially a single sleeper has a quite expensive pass reservation on Trenitalia night trains. If any saver fares are available, then those are often cheaper than the pass reservation. Only the full fare ("base”) is more expensive than a pass reservation for a single sleeper.