My understanding is the substation is actually a fairly normal part of electrification of a bus depot, because bus depots need a high volume of electricity they need to be connected to the high power lines (charging a single bus via domestic connections can take days) but this needs to be stepped down to be suitable for charging - in the case of Centrebus I understand the high power line that needed to be connected to was actually running down the road outside so the connectional issues often experienced were minimised. It doesn't necessarily need to be on site but it does need to be close to reduce installation costs and minimise power loss as this is greater at lower voltages needed for actualy charging. The issue, I believe, for Centrebus and Arriva at Thurmaston was around getting the landlord of the depot site to agree to the location of this vital piece of infrastructure as the national grid look for relatively long term guarantees of substation location (50 year leases for instance) which mean landlords may not wish the equipment to be in the best site for the operator as it then becomes a restriction on what can be done with the site if the bus operator leaves. This equipment is not always obviously located and not all of the necessary equipment is clearly visible.
The big issue that operators find with electrifying a depot is that the high power line that they need to be connected to is a distance from the depot meaning the connectional costs can run into the millions which can be difficult to fund. You are starting to find 3rd party suppliers entering the market that will fund and install these connections on the basis that will now be a long term demand for these connections they can charge for generally as electrification becomes more generally widespread. The other issue operators running out of older depot buildings can find is that the building simply can't accomodate the charging infrastructure due to the stress on the fabric of the building and/or the space that has to be taken up to avoid it. With Arriva Tamworth it could be any of these or a combination of all of them, the site is very constrained due to its town centre location that means it may be difficult to make space for the connectional requirements, it may be some distance from the suitable lines with high enough power and the building may be difficult to install the charging infrastructure in.
I think part of the reason for First moving the depot was the old Abbey Park Road site remained owned by the council when the operator was sold, it may be that it wasn't worth buying at the price the council wanted given the costs that would have to be incurred to bring it up to standard and it very quickly was far larger than the operation needed but the site was big enough that they could have done the redevelopment in stages without moving off site if they had actually wanted to.