Hi @mja,
For now this is the only workaround we have. Are the results satisfying?
You could also use PIX4Dmatic which supports the OSGB 1936 / British National Grid - EPSG:27700.
Here is also a guide on How to vote for your favorite Pix4Dmapper feature requests/suggestions. You can create a new topic and vote for it. This way it will be registered as a feature request and it can get upvoted from other users as well.
Thanks a lot for your feedback.
Best,