The “London Beth Din” is under the UOS, which means it is under the Chief Rabbi.
There is zero heter to enter a Church sanctuary. His predecessor claimed he did so “mipnei aivah”. As others noted, why not permit everything else “mipnei aivah”, too?
If there were any serious halachic permit to do this, then a teshuva should have been published, especially because the Chief Rabbi studied in Religious Zionist institutions and that faith is idolatry according to Rav Elchonon Wasserman HY”D and the rest.
Ironically, assimilating with the gentiles is, of course, a generator of aivah, as we have seen throughout history, R”L L”A. So, if the goal was to prevent aivah, then the most prominent Jew in the UK attending the most prominent church in the UK was certainly not the way to accomplish that goal.
So, the working assumption by anyone who at least wants to be G-d-fearing, is that this was a tremendous chilul Hashem.
That’s all in addition to Neville’s post here that the videos and publicity is certainly a slap in the face to G-d even if there were some real heter – and there obviously was no heter.