As I said, I'm not familiar with the watchguard setup. The way I'm reading it, it looks like it's just as likely to NAT 10.x.x.10 -> 86.x.x.10 as it is to NAT 10.x.x.10 -> 86.x.x.200.
Given that it seems you're leasing the /24 from BT, you may find that it's easier to see if they'll route the subnet to you so you can just assign the publics to the hosts themselves (if that's really what you're after -- though, if these are just workstations, assigning them publics / using 1:1 NAT is kind of unnecessary in the first place ...)
