Stay home and save the rest £200.
Prepaid are normally a better rate, and despite the T&Cs prepaid stays have always counted regardless of when the actual stay is. It certainly worked for a recent bonus points offer on a different stay I have next year.
Only valid until end of 2018 so I would not cancel. Unless you get a better rate.
Hilton is on my BAPP and Platinum (y) 🏻