I fly night OCAS much of the time, increasingly so now the days are rapidly shortening as winter draws in.
In reality, despite what the books say, a paper flight plan isn't worth much for a flight like this, mainly in Class G airspace. If you are concerned about SAR in case of a problem (and you should be, especially on your proposed route) in practice it's far better to ensure you remain in contact with an ATC unit at all times. If there is no-one else more suitable then just use London Information, who are forever helpful, God Bless 'Em. Also, let a responsible person know your route and timings and check in with them after landing. Agree that they will take overdue action if you don't check in. If you operate from a flying club, it goes without saying that this should be done.