is it realistic for me to fly such a distance (400nm each way) regularly given my (shallow) depth of experience, bearing in mind the channel crossing?
Of course, though as Fuji says you are not going to get something for £20k which you would want to be doing this in - even if it has a 400nm range which quite a few won't have.
And if you buy an old dog, you will be throwing huge amounts at it in maintenance.