I cannot see the issue here. To my simple mind the HSTO forms part of your 'landing roll' and as such you have 'approval' to use it, whereas a taxyway does not. Therefore you MUST plan to be able to come to a halt on the HSTO. Whether you do or not will depend on getting permission to use said taxyway in time (except, possibly for 'FR' on 26L at LGW). The Mark I eyeball is a valuable asset in determining the future.