I brewed something similar last year. I ended up at 1.060 after getting much greater efficiency than I'd calculated or planned for. The 1968 ESB yeast took it down to 1.018, about 70% attenuation. However, I mashed at 156°F for a full bodied beer. Your lower mash temperature should make the wort more fermentable, so you might get another point or two out of it, so 1.015 would be about right.
My only worry about the temperature is that on the low side of its range this highly flocculant yeast might drop out too soon. Your temps inside the fermenters, though, will be a good 5-8° above ambient just from the heat of the fermentation. Like you, my inclination would be to leave them alone for the first 5-7 days then take the fermenters out of the basement and put them in a very slightly warmer spot for 3-4 days to make sure the yeast has a chance to finish up.
Wyeast 1968 and Safale S-04 are both reputed to be the Fuller's strain, so it will be interesting to taste them side by side when all is said and done. I can tell you that the rich, malty beer that I ended up with was one of the best English ales I've brewed to date. I suspect yours will be pretty darn tasty as well.
Good luck. Keep us posted.