If we think of how the WCML is laid out and where the TASS function is used and to what extent it speeds journeys up, it should hopefully become increasingly obvious that tilt on HS2 and future WCML stock isn't really a necessity.
Let's start up at Glasgow, the whole line from Glasgow - Carstairs has no TASS function, bar a short section of TASS on the down main on the straight between Law Junction and Motherwell through Shieldmuir, where the track is TASS enabled up to 105mph (for a short time of this, the conventional PSR is 105 before this reduces to 90 EPS105). Say that gives us 2 minutes of running at 15mph faster on this straight piece of track, that means a tilting train would be running approximately 0.5 miles ahead of a non-tilting train, which at speeds of around 90mph, gives you a time saving of roughly 20 seconds). 20 seconds worth of saving on the entire 25 miles between Carstairs and Shieldmuir (and only in one direction) does not justify the cost of a tilting train.
From Carstairs - Lockerbie, the line speeds vary greatly. Between Carstairs and Lamington, the linespeed is 125 throughout (reducing to 120 over Lamington viaduct), and this is pretty much always 15 mph above the conventional PSR. Short section through Abington has 90 with no EPS, past Crawford there's an EPS of 105 up to 110 (which is again 10-15mph above the 95 Conventional PSR). Descending down Beattock tilting trains have to follow conventional PSR's for quite a bit of their journey down, starting at 90 then working up to 95 and 100. Around halfway down, there's an increase to 125 EPS, which is 25mph faster than the conventional 100mph limit down the whole of Beattock (although, tilting stock does admittedly have to slow down towards the bottom of Beattock for a further 100mph PSR in line with conventional trains). On average, over this section, tilting trains run around 10mph faster than conventional trains, meaning that a tilting train would be roughly 3.5 miles ahead of a conventional train coming out of the section, which with an average speed of (roughly) 100mph, gives us a time saving over this section of around 2 minutes.
Between Beattock goods loops and Lockerbie, EPS trains can run more or less 125mph throughout (there's a short 115mph EPS section immediately after the loops), with conventional trains running at 100 initially then up to 110. This section of track takes roughly 5 minutes to traverse so let's assume that if tilting trains can run (on average) 18mph faster than conventional trains on this section, they'll be approximately 1.5 miles further ahead than a conventional train coming as they pass Lockerbie. At these speeds, that's a time saving of roughly 30-60 seconds.
Between Lockerbie and Carlisle, the speed limits are similar to those between Beattock and Carlisle, with conventional trains being restricted to 100 between Lockerbie and Quintinshill and EPS running initially reduced down to 115 then moved up to 125 just before Ecclefechan. Let's say that gives an average of 20mph faster than conventional stock for a tilting train over this section (which lasts around 10 minutes), that means a tilting train would be roughly 3.33 miles ahead of a non-tilting train, which would give a time saving on this section of track (against a speed of 100mph) of pretty much exactly 2 minutes.
The final approach to Carlisle from Quintinshill sees tilting trains running at 125EPS which goes against a 100mph conventional PSR pretty much the whole way before both trains have to slow for a 55PSR down to a 20PSR into the station. The EPS section takes roughly 5 minutes to traverse before slowing for the 55PSR, so let's say this gives us 5 minutes running 25mph faster than a conventional train. This would mean a tilting train would be roughly 2 miles ahead, which in this case is a time saving of roughly 1 and a half minutes.
So in total, that means (on average), a tilting train will arrive into Carlisle around 6 and a half minutes earlier than a conventional train running non-stop. Is it worth the cost of installing and buying tilting mechanisms just to save 5 minutes on the entire Northern 100mph of this line? I would argue it isn't. If we compare the timetable of a 390 to a 397 travelling over this section of track, both of which stop at Motherwell and Lockerbie (to account for the time taken to slow, stop, and speed up to line speed):
Realtime Trains provides live realtime running information for the Great British railway network using open data.
www.realtimetrains.co.uk
This is a 390, which between Glasgow and Carlisle (stopping at Motherwell and Lockerbie) takes 1 hour and 14 minutes.
Realtime Trains provides live realtime running information for the Great British railway network using open data.
www.realtimetrains.co.uk
This is a 397, which between Glasgow and Carlisle (stopping at Motherwell and Lockerbie) takes 1 hour and 24 minutes.
Now I'm entirely aware that the TASS function gives a much greater advantage over speed on the south WCML (indeed the 390 EPS is 125mph all the way from the North end of the Watford tunnel to Stowe Hill tunnel, well over 50 miles away). However, bearing in mind HS2 trains will not be running over this section and will only join the line at Crewe, the majority of the time saving (with tilt) would be done between Crewe and Preston (the tilting saving between Preston and Carlisle is similar to Carlisle to Glasgow). I would not argue it's feasible or sensible to spend a considerably higher amount on tilting stock for such a short saving, which compromises on space, passenger comfort (390's are very claustrophobic compared to some non-tilting stock), and adds such a huge cost onto a project which will already cut journey times by unprecedented amounts between the North and London.
The benefit of experience.