I'm rather curious about whether it's ever been done... has anyone ever gone from a first class external LLB (graduate route or otherwise) from the University of London into the Oxford BCL?

Is there any chance of being accepted to the BCL with that particular qualification?

It's possible, but I'm not aware of anyone who has managed to to do that. I do know a few people who were accepted at Cambridge after getting firsts in the UoL external, and quite a few people who were rejected at Oxford despite having firsts in the UoL external. But I would strongly encourage you to apply if you have some really good references (preferably by professors or doctorate holders). Otherwise it might be wiser to take a vocational course before applying.

I have known of two persons who received first class honours on the external LLB and then went on to do the BCL at Oxford. Plus another who went to do the LLM at Cambridge. So, indeed it is possible. I myself am pursuing the external LLB and hope and pray I will be able follow that path one day in the future. Awaiting my Finals Part 1 grades will tell me how close, if at all, I am to that dream.