Try asking the Vintage Sports/Car Club of the UK. I am sure they will know several sources.
I had my Bosch D2B done by a German Engineer, there is quite a bit to the process.
I’ve had one rewound with new condenser in the UK. Ribblesdale Auto Electrical in Preston. They did a good job on that part of the mag but couldn’t source new bearings, needless to say two years later the bearings started to fail. I sent it to BMZ Magnetzünder as link above and they sorted it. The bearings are shimmed, insulated from the case and need special grease so I thought best left to the experts. They also fitted a new dynamo armature in another and while it can take a while the job is good. I will go straight to them next time.