I suspect battery life combined with "dead mass" would limit their use to BP only regardless of AUK rules.
I believe JennyB OTP has form for this sort of thing. At the current state of technology, on longer rides it becomes a non-trivial challenge in battery management, not dissimilar from the challenge of managing sleep.
Yes, I tried the first Mille Failte and only managed the first day, mainly because my night navigation was crap. Report
here.
At that time my fitness level was such that my long distance rolling average was about 20 kph on an unassisted bike, and around 25 kph assisted, using about 5 watt hours per km. I was carrying 20 amp hours of 36 volt battery which at that usage gave me well over 100 km between charges, so the plan was to take two rest, eat and recharge stops per day of a hour each, and recharge again while sleeping. I was using two 6 amp chargers in parallel, feeding directly into the battery. The trick is to use a Cycle Analyst to check how much current you used, and time the charging to stop when you reach 80% full. That way you don't go through the lengthy balancing phase, an hour will give you a full 12 amps
Needless to say, this is not the kindest way to treat a battery. I never had a bms on that battery, and now, with 4 years of abuse later, it's down to about 8 amp hours. Get the biggest battery you can afford, and charge it as slowly as you can bear, and only charge as much as you need to.
In short, with proper planning an ebike is a definite advantage over 200 km or so. Beyond that, there's probably no time advantage, but you'll get another hour or two of rest during the day. When cycletouring I normally bring the battery in and recharge at cafe stops. I've never been refused yet.