The information is simple, they will not deliver to a PO Box in the UK!
We needed to send important papers to the address of an investment company & used Chrono Express for the fastest & safest-cost me €48!
Posted on Saturday 17 April 2016, finally got collected on 18th & currently being held due to incomplete address! My lengthy wait on the phone revealed the PO Box problem that the post office failed to mention on Saturday.
Nothings ever easy/simple in France maybe?
All the best,