Not necessarily.
Conventional electric units have to draw power at the instant that it is used, they have to do this regardless of system conditions.
A battery train can, to a reasonable degree, determine feeder conditions using the proxy of line voltage and adjust its draw of power from the system accordingly.
A system dominated by battery fitted trains could load the feeders far more heavily without risking an actual overload and trip.
However, only up to a point (and as you intimate by saying not necessarily there's some element of doubt), as if you added more trains to being battery trains they will need to be able to charge somewhere, so you'd need to be sure they could do so, even if that means in theory there's still spare capacity in the system.
For example you've got a line with 2 bimodal trains and 2 DMU's by converting all of those to battery trains there's still going to be a need for traction draw for what were the DMU's in addition to the battery charging. The loading there could be managed better by having batteries, however if you need to be sure of a certain account if charge and so an extra feeder is needed, arguably it's better to do so in a new location (more electrification) especially if that then allows other services to move over to being battery trains.
I don't know the MML, however it's not unreasonable to assume that to get the Matlock trains to be battery trains there's a fairly good case to be made to have OHLE at Derby, especially given that would also aid other services (including XC).
As such, why not just get on with the electrification of the MML as it would aid with the case for battery trains?
If it turns out that too much was done, then whilst there'll be extra cost there's still the benefit of extra resistance.
If we were discussing the electrification of the gaps between Reading and Gatwick, I could understand the talk about batteries being suitable and let's hold off, however we're taking about a line which has a lot of big gaps and creates a key bit of infrastructure to allow far more services to move to being battery trains, as such let's just get out done.
It's similar to if the discussion was about between Exeter and Plymouth, that would be key to delivering battery trains on the branch lines near there as well as reducing the length to a point where long distance services become easier to deliver as battery trains and so should be done.
Could there be savings by missing a section, quite possibly, however unless there's a few expensive bridge replacements and/or the gap is a long distance the extra reliability it provides is probably worth doing anyway.
One of the key advantages of battery trains is the fast that they can keep going if there's an issue with the supply. However, you can't rely on that being an advantage if you are squeezing every last mile out of the range of the battery trains be skimping on how much electrification you provide.
For example, a train (with a 140 mile range on batteries) it is better for that train to reach OHLE at the 110 mile mark of it's journey so if there's an issue there with the power it can still go another 30 miles to find power. Especially if the alternative is to run it to 130 miles and only have 10 miles of range left, as basically that train is making it to the next station before stopping to await the power to be restored.