Bainbrig 14.33. Clearly if the issue only comes up for consideration when the person is entirely unfit to travel then that is out of the question, but this is the only option in many countries where laws prohibit assisted suicide. However, when travel is to be considered there are other ways than flying (and it is possible that no airline will accept the passenger). My last travel option would be a slow progress to the nearest country permitting assisted suicide, by road and making as frequent stops along the way as necessary.
Personally I would not really care where I were to do the deed if it is proscribed where I live - but properly framed law permitting assisted suicide is always most likely to involve licensed institutions and individuals and that will normally preclude them coming to you.
Because of the tendency for procrastination, obfuscation and general can't do ("want to but, gosh, it's so complicated, have to take everything into account, consider all the.....") assisted suicide is almost certainly a long way off from being legalised in the UK.