If you look through the British forums there are many examples
https://www.lfgss.com/search/?q=rotrax&defaults=true
https://www.retrobike.co.uk/search/1...ax&o=relevance
and there is an excellent Facebook group Friends of Rotrax cycles (sorry but this forum has a problem with FB links)

I encourage you to read through Quiet Mike's 1948 Rotrax - latest project. It is a wonderful story and so beautifully photographed.
You may notice my suggestions in that thread.

An important consideration in bikes of this period is the project's end goal. If you are aiming for a period-appropriate bike, it may be far cheaper to buy an unmolested original bike rather than a frame and then buy the parts separately. The cost of the postage for multiple packages to international destinations are needed to be factored in. I live in Australia and my bikes have period-appropriate components and a number are British (1946 Hobbs of Barbican, 1952 EA Boult and 1952 Hobbs of Barbican Blue Riband)