Hi Roger,
My wife and I came out on the E600 visa in Dec 2017 having already applied for the 143 visa in May 2016. On arrival we applied onshore for the 864 visa, which entailed cancelling our 143 application and paying another application fee. That was quite easy as all the previous documentation from the 143 was acceptable and did not have to be repeated. I have not been given a bridging visa as yet but I have had two separate notifications about a month apart. One confirmed that my 864 application was accepted and the latter confirmed that my 143 application had been cancelled due to my now current 864 application.
Regards,
Jack (Bilby47)