Is your granny flat going to be a ‘proper’ address in its own right , so liable for its own council tax , its own official address with Royal Mail etc, ?.
Assuming it’s not a separate address, If you want a second FTTP service , the correct process would be for the ISP you use for that service as part of the ‘order’ , to arrange for a multi port ONT from Openreach to be supplied, you may find this less than straightforward, but that would be a problem for you and the ISP you chose , the ordering system is common to all ISPs , some are better than others when knowing how to use these systems, some ISPs only show their staff the basics , as that’s all they intend to supply.
The Openreach physical installation then becomes mainly a change of ONT , the configuration work etc done by the ISP and OR remotely and in part in advance of the visit , but no extra cables from the CBT or CSP are required, both services are on the same ‘fibre’ , the first service on port one , which would require the existing service to be reassigned to the new ONT port one , the new service on port two , so both routers would be as close to the existing ONT position as the length of the Ethernet cable supplied by the ISP with their router , any extra power outlet needed would be your problem, as the new router obviously needs a power supply as well as the ONT and original first service router.
If you wanted the router of the second service inside the granny flat , you would need to provide the means to do that yourself, either a long Ethernet cable or Ethernet cabling and sockets , the ‘connection’ between the ONT and router isn’t optical.
There is some suggestion that a second order would be a new connection from the CBT , possibly a new CSP and an optical cable ran from the CSP ( existing or new ) to a new ONT location and a second single port ONT fitted , pretty much where you would like the service to be , but the way the OR network is dimensioned ( one CBT port per dwelling ) an order for a second service shouldn’t be progressed that way ( one reason why your chosen ISP may struggle to raise an order without mistakenly simply taking over the existing service )
