I believe they were modified so that they could work more effectively in multiple with the other 350 subclasses which work at 110mph.
Hence my suggestion up thread of their use by XC on the ECML and into Glasgow and Edinburgh to allow the strengthening of the rest of their fleet by the released Voyagers.
It would provide them with a noticeable uplift in capacity along the routes served by the 350's as well as allowing more diagrams to be strengthened.
Even if you only create 4 spare units they could benefit quite a few extra diagrams (between 8 and 12 or more) as either you release a 4 or 5 coach unit:
A 4 coach unit released would enable it to run with another 4 coach unit, however chances are the increase in capacity would be better suited on a diagram currently run by a 5 coach unit. As such that's at least 2 diagrams which benefit.
However it gets better still if you release a 5 coach unit as that unit can replace a 4 coach unit, which then in turn could pair with another 4 coach unit and repeat the same thing as above. In doing so that's at least 3 diagrams which would benefit.
The reason that it's at least 2 or 3 diagrams benefiting is that one unit could be turned back before the outer edges of the network to provide extra capacity to more services than if it ran the whole length of the route.
Given there's the potential for it to provide some extra capacity on 12 out of about 55 diagrams (there's 58 Voyagers and 5 HST's units but there'll be some spares and some doubling up already which reduces this number from the total number of units available) if it released 4 units, then that's going to ease pressure on capacity quite nicely.
The reason for picking 4 units is that there's 1tph which runs to Edinburgh with half of these going to Glasgow (assuming that the 221's carry on running to Newcastle with the 350's overlapping with them between there and Leeds/York) then the round trip time is 3 or 5 hours, this would require 4 units (assuming no pairs of units run any services).
Whilst this would only require 20/21 units if every service ran to Glasgow and was run as an 8 coach train with 2 or 3 spare units. As such there's still scope for additional TOC's to use the remaining 16/17 units.
[WISFUL_THINKING] That is unless someone were to electrify through to Birmingham, then XC could use the whole fleet.
[/WISFUL_THINKING]
Given that ScottRail have similar it could be convenient for them to maintain the units for XC (so as to limit the extra costs to XC for their maintenance).