Actually don't bother as I tried. All the reports have different values. There are million ways to account one's arm exports, and there many countries actually try to hide their arms import/export. The arms export has always been a controversial subject internationally. Unless you can get the report from UK's Parliament and the exact contracts from each arm exporter and private arms dealers(legal and illegal), there is no way we will know the true volume.
I had a inkling this was the case reminds me of the the UK-Saudi deal back in the 70's marred in scandal
BAE and the Saudis: How secret cash payments oiled £43bn arms deal BAE admits wrongdoing after US justice department lays out how British firm used intermediaries to hide money.
BAE and the Saudis: How secret cash payments oiled £43bn arms deal | World news | The Guardian
I dont think we will ever know 'actual' figures of arms exports.