200 EUR/Hour
I think the 200 GBP/hour is a bit much, I would estimate closer to 200 EUR/hour or roughly 2 EUR per NM. I actually did precisely what you are describing in a 172M at intervals before buying my own plane. Now I think of the cost in fuel terms--roughly 100 EUR per hour (depending on where you buy your fuel). As people have said, you will have an issue getting a rental for that long. Your best best is most likely not a club or flight school, but private owner.
If you can find a mogas burner, that will help you on costs.
And, as others have said, you'll remember the trip forever!