I know that problem and I've been told also that it depends on whether they thought to register you on the electoral role or they forgot to ask when you registered at the consulate. This is unfortunate that it is not mandatory to tick a box saying that you do or you do not want to be enrolled and they should review their process for the future.

However, for their case, I would say that they do not have enough funds in London to manage the biggest French community abroad. They recently made huge improvment to their queuing system allowing people to book time on the Internet.

It is also a shame that you did not pay attention to where you were registered in time. A proxi vote would have been possible a few weeks ago.

 

True, maybe they should have sent a specific mail to everyone registered at the consulate in order to make sure they are registered/know how to be registered/know how to vote.

 

That's the big problem of Consulates within the European Union. Consulates receive funds in proportion of their registry. A few weeks ago, I was at a conference where the senator for French citizens abroad said that it was quite a good thing that people do not register that much within the EU, which shows that they feel confident enough to be a European citizen, not just a French one.
But the bad consequence is that French population abroad is underestimated in Consulates in the EU, and therefore that those Consulates are under-funded and cannot provide the service they should.