bt did suggest a fibre without voice at £57.50 a month!
Would they charge you a reasonable price for a *brand new* FTTP+voice service (with a new number), as opposed to migrating existing service?
If it's reasonable, then simply keep the Waitrose service running in parallel. Use the old copper line for voice calls.
Wait until the new FTTP service is installed. Run in parallel for a few weeks. Then when you're happy that the FTTP service is up and reliable, *then* you transfer the phone number to VOIP, which (as you say) will cease the old phone line and the attached ADSL service - which is what you want.