Try putting the routes in using ABRP (A Better Route Planner).

Occasional 400 mile days for most BEVs is no big deal, especially if you have chargers on both ends.

Even with a household plug for the first couple months, your car will slowly get closer to empty over a week, then charge back to full over the weekend.

With ABRP, you can plan out a whole week with slow charging at home, faster charging at work, and then see where and how often it needs you to stop for fast charging.

It depends on the details of the car scheme. A BEV is great, assuming you can charge it cheaply at home, except for the two days where you have a 400 mile round trip. For those, motorway fast charging will get you there and back. I have had a company BEV for 3 years. Worked out ok with lease cost and tax benefits. But I have now decided to abandon the company car scheme and got a private car with a petrol engine. Never saw the point of a PHEV, lugging a heavy battery around while running a small petrol engine. Probably good if you mostly drive around town, but that’s not my use case. For private cars, BEVs are just too expensive, to buy, to insure and to repair. My reasons include:

I get a cash alternative if i don’t take a company car,

I can’t claim actual cost for motorway charging, which is very expensive. It works out more expensive per mile than a petrol car (for me, a small modern BEV may work out better) This is the main issue for me as I do long trips fairly regularly.

My car needs 2 hours to charge even on a motorway fast charger. Normal local public chargers don’t work for me, too slow. The car needs 12 hours on my 7kw home charger.

She's going to be able to expense charging when doing these runs, so the only issue with public charging is time / convenience, right?

I'd always go BEV, because I already drive one and would never go back. I know that's different to most people though and the thought of a BEV is scary to some. It will be the better car to drive without a doubt. PHEVs tend to have low power engines and small batteries / motors in order to make everything fit. It's a compromise car. A BEV will have a full size motor and battery because there's no engine to also fit in. Servicing will also be a lot simpler if you care on a company car.

On the days where she's going to a site with a charger there should be no problem (as long as there are enough chargers or she can reserve one). Drive there, plug-in, work, drive home. No petrol stations ever.

At the sites without you could look in zap map to see if there's anything nearby. You're looking for 7, 11 or 22kW chargers with a type 2 (not CCS) plug. If there's anything within walking distance then it's the same drill. Just be aware of any parking charges.

Alternatively you're looking at high power charging at motorway services or similar. On that 420 mile round trip I'd suggest it's probably a stop on the way out and one on the way back. Maybe 20mins each way. Again zap map or even better "A better route planner" (ABRP) will help you find chargers. (There's more than you think).

Don't be the owners that think the range is limited to what you can do on a full charge from home / work. Be comfortable with public charging, just like you're comfortable refueling your current car. It's different, but not scary. Just give it a go somewhere local before you NEED it to work on a long journey that became a little longer than expected because of a road closure or diversion.

Biggest tip is to consider which car charges the fastest. This may not be the car with the highest headline figure, but the one that sustains a good rate for longest. This is where reviews are good. People like Tesla Bjorn on YouTube do a lot of charging speed comparisons and keep tables of this stuff if you really want to go nerdy. ABRP tries to bake it all into the calculations, so comparing different cars on the same journey should give you an idea.