Hard to tell since it is only available in Apple Stores but I would have thought at least still available until the 15th or the 22nd. Also it is called 'iPhone Payments'
If you are going down this route there may not be a huge difference between the 7 and 8 per month in which case if I were you I'd get the 8+. I am currently paying £43.50 per month for my 256gb 7 plus and the 256gb 8 plus is £44 per month................though I appreciate the 7+ will be less since they dont have the 256gb model available.
Olly W is correct went to the Covent Garden store and the first person said she thought it'd still be available at the
moment but would need to check with her colleague but then he said no it's now only available on the new phones 8/8+/X