You could try and see if a flat to post mount adaptor would work? Print one out and check?
https://www.assolutions.ca/flat-mount-caliper-to-post-mount-adapter-guide/
With postage from Canada that comes to about 35 pound and for 15 more I can get a fancy Hope brake. A matching rear to which I was going to buy anyway
They do have a UK distributor...
@PhilDAS started
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You could try and see if a flat to post mount adaptor would work? Print one out and check?
https://www.assolutions.ca/flat-mount-caliper-to-post-mount-adapter-guide/