"Although there are people who commute up to 30 miles each way by bike it is unusual and
a 25 mile round trip is on the outside bounds of what is realistic on a daily basis. ..and still do a days work. "
Is it? I have a 25 mile round-trip to Manchester City Centre each dry day and it's just 40 minutes each way. It's more restful and qucker than squashing onto a train and walking down from Piccaddilly to be honest.
Mine is a flattish commute too and the only issue I have is that I could do it more quickly and probably without raising the heart rate much more on a much lighter £600 gravel/cross bike - like this one from
Planet X, for example. My Cube Touring is rock solid but weighs a ton and it's not really worth the effort of getting it up over 16mph.
If you must have an ebike for some reason, on a £1000 budget I would look at the Woosh bikes or a Woosh conversion kit.
And then fit Marathon Pluses. You could get some decent weekend away days on that.
Once you get to £1400+ you can start thinking about Bosch ALP mid drive bikes (they'd need to be on sale at that price point but there are a few around) but that is likely overkill for a simple commuter.