Thanks to all for your knowledgeable and timely responses. Were it for me i would indeed probably buy a ready-made bike or better conversion candidate....but my wife love the Britannia - colour, style,basket, quality etc.
Am minded to look at the bottom bracket fitment with rack battery mount and so lose only the chain guard (possibly adapt / replace) I guess rather than wider conversion options.
If so what would the Bafang BBS01B 250 be the way to go on a bike of this weight?
Thanks all.