BTL application at DIP stage-wait to remortgage residential mortgage?

ran123ran_2

I am looking for some advice. I have just got DIP for a BTL mortgage on a small flat. I can see the lender has done a credit search on my file. I have sent copy statements, etc but everything still has to be reviewed by underwriters.

I'm also looking to move my residential mortgage to a new lender.
I went through the application (all paper based - not system based) before I did my BTL but had asked the mortgage advisor not to send on the application as I didn't want any credit checks to hinder a potential BTL mortgage. If my offer on the BTL property didn't get accepted then the mortagge advisor would have sent the application.

Can anyone advise at what stage I should continue with my residential remortgage?
Once I get a formal BTL offer?
When I have accepted the BTL offer?
Once I have completed on the BTL property?
Or just do it now?

Thanks.