Interesting. I would be careful especially if your bike isn't made specifically to house this kind of electric motor and battery. Given it won't fit in your seat tube, it might be a blessing.
if you're willing to pay £2k to electrify one of your existing bikes then you may as well consider paying for a custom built one. You'd get a pretty decent one if you're willing to to pay a bit extra over and above that though. But depends what you're after.
I have a mid drive motor on mine and the battery sits within the frame; but it's purpose built.
Mid drive motors, are the most balanced given that it adds power to the drive train. Rather than hub motors, which I have read are separate from the drive train and are either good on flats or uphill but not both. Hence why I went with the mid drive. But it does make the bike heavier.