Milan-Zurich can be done reservation-free by changing in Lugano. It takes the same time. Or you can save €3 by booking only the mandatory section to Chiasso.
All reservations mentioned can be done on raileurope.com. Always change currency to euros, or else you'll be paying 4-5% more.
Also you don't have to go through Zürich. Chiasso/Lugano-Basel is now a superb 2 trains per hour and the through ones run via Luzern. TER Basel-Mulhouse then you only pay a Eur10 domestic TGV supplement rather than the Lyria 40+(?) in first from Switzerland. Milan-Lugano on the R80 avoids the italian Eur13 supplement and is just as fast.
Just to second comments about the Swiss leg. Use walk on trains (just like the InterRail of old - yeah) from the first point your journey will meet Swiss trains (Milano) as far as the last point where your trip will use Swiss trains (Basel). I wonder if you have made the common tourists error of assuming that to get a TGV to Paris from northern Switzerland you have to go to the starting point (Zurich), when in fact the train just runs East across Switzerland to Basel before heading north (and it is not unknown for the Basel -Zurich - Basel leg to be caped anyway and replaced with an 'ersatzzug' (emergency replacement train) .
Another option is Milano - Locarno (stay at the Ibis); route IR26 train 'Treno Gottardo' Locarno - Gotthard mountain route (spirals, Wassen church et al) - Olten; EC 6 loco hauled Eurocity Olten - Basel SBB - Karlsruhe; TGV or ICE Karlsruhe - Paris Est
I want to do the Milan - Switzerland journey in daylight (for the scenery) and the day is going to end on Eurostar to London. I had considered changing onto the Lyria at either Zurich or Basel. Since this is a holiday trip with my better half, the fewer changes and more relaxed boarding at the terminus tipped the plan to Zurich. Depending on the exact timings in this September's timetable, that might change.
But what I hadn't considered is going on to Strasbourg instead of taking the Lyria and thereby arriving at Gare de l'Est instead of Gare de Lyon. Thanks to you all for opening my eyes to the further options from Basel.
Taking that idea further, the timings seem to work to get a TGV from Strasbourg to Brussels instead of to Paris and change at Lille Europe into Eurostar. That is attractive for the easy changes, and especially if we can get a direct Milan - Basel train. EC 60 fits the bill, but goes via the Simplon (I think) and I know that there is a prolonged period of work on that tunnel but I have no idea how disruptive it is likely to be.
Finally the old Gotthard route is very appealing, but I don't think it can be done as part of a daytime Milan -London journey.
So, thanks to you all.Any further thoughts, info or suggestions are very welcome.