Oh this is easy to solve, you can have through journies and the ability to cope with loadings, don't advertise the service as calling westbound at Reading and maybe not even Newbury, have these as pickup/setdown only services.
But yeah... Overall I recon the 180s should be used on the B&H Line for Exeter / Taunton semi fasts from Reading / Paddington with some loading controls in place to prevent London - Reading Passengers knackering up the works should they be extended into Paddington from Reading. It's somthing they'd be suited to, since IIRC theres a bit of 110/125 on the B&H line?
Either way, this is going to be the route with the most 100mph+ running, in the biggest requirement of LDPE / LDIR Stock, with the least amount of diesel under wires running. Unlike other routes on the GW that are wired until Oxford, Swindon, Cardiff (And if Cornish services run via Bristol) Penzance. The services running via these destinations to be served by Electrodiesel Stock rather than Diesel stock.
And of course, with my current view on the Bi Mode Hitachi SET I wouldn't want it to be these, seeing the bi-mode part of the project scrapped and the carrages made a more sensible 23m long to fit within the C3 Guage profile (the same as Mk3s) and become an EMU Capable of easilly being hauled by locomotives, with ETH supplyable either from said locomotive or backup generator (NOT USED TO GENERATE TRACTION CURRENT, ONLY ETH!) so that ETH is availale in wires down.
Said locomotive could work in push pull with either driving cab.
The EDMU Requirements of the GW where changing from Electric to Diesel will not have much space and will need to be done rather quickly will be forfilled by the conversion of the 222 fleet cascaded by the electrification of the Midland Mainline, extra 180s provided from TPE Electrification and Electric SETs being provided to FHT. When additional EDMU capacity is required, this can be provided by the electrification of core XC services, wiring from Oxford to Coventry first, releasing 8 Class 220 units, then, combined with parts of the GW Electrification, Birmingham to Bristol. and finally, Birmgham to Reading.
If additional units are required at time 0 for the GW EDMU requirement, additional 220 units can be cascaded from XC by cascading 221 units displaced by additional 390 units from ICWC, resulting in about 10 220 units and the 27 222 units with GW at time 0 of moving from HST to EDMU / EMU SET operation. All these 222 and 220 units would be extended by some cars, either 1, 2 or 3 to become EDMUs, the same treatment to XC's fleet and the ICWC fleet to make the entire 22x fleet into the 22(x+5) fleet? 226, 227, 228? As EDMUs..
Personally I'd say.
4 car - 5 or 6 car, depending on the amount of time off wires at >110
5 car - 7 car
7 car - 10 car
Possible reforms so that 6 car units can become 8 or 9 car units. But 10 car 'Jumbo' units would come in handy for the far end of the GW Network that needs a lot of capacity.
PS: SDO is natively supported on the 222 fleet so it's also suitable for operation over the kemble link etc.
With these 27 222 and 10 220 units available by only electrifying the MML and ordering additional 390 units for ICWC there would be no need for a Bi-Mode SET. Especially if wires are exteneded to Swansea.
Wires are likely to be extended to Aberdeen and possibly Inverness soon anyway. If the SET provided loco haulage as an option ICEC could use this, or sacrifice some or all of the inital allocation of 220 units to ICGW to ICEC for use on the top of scotland and other off wires services (Hull would be wired as part of TPE N), Aberdeen wired soon aswell (Proberbly by 2025 at the latest) leaving only Harrogate (May be wired soon) and Inverness, and Lincoln if it's added as off wires services. 10 6 car units that could leave the cross coupled and split on route would do just fine for ICEC.
When GW do need more capacity, this could then be provided by electrification of the GW, XC (Oxford - Coventry) or other parts of the network to release 221 or 220 units, with the preference always being sending 220 units to other operators.
Idealy, additional 390 (or 391 units) would be designed to be Pull Pull operable with a locomotive for services beyond Crewe / Chester. Then all 221s to XC, released 21 220s or 221s stright away for GW/EC.
Anyway, off topic enough yet?