Aren't some of the rides out of York flat(er)?
Obviously, every 200 in Essex/Suffolk/Norfolk is a (relative) piece of cake, climbing would be around 1800-2100m and the hills are at worst short and steep, mostly very benign.
If you're really going to struggle time-wise, I'd suggest leaving a 200 till you're faster. There's lots of 100ks with lower minimum speeds. Then at least you can be motivated by your success rather than intimidated and put off by being out of time.