Hi RobFrain,
Could you please elaborate your issue a little more ?
You are using a 3CX FQDN (bpsc.3cx.co.uk) so you choose option 1 "I need a 3CX FQDN" then you enter "bpsc" (the fqnd you choose) and then option for suffix 3cx.co.uk . Installation will not ask you for path , it will automatically generate a certificate from Let's Encrypt .