No Snake Plissken, thankfully the average UK humidity falls at about 45%, so you should be fine. However, the ambient humidity outside your flat (Richmond, Surrey yeah?) is 71%, but the humidity in buildings is usually kept constant.
Where do you keep your snakes? If you keep them in a room where things will get hot and stuffy (e.g. laundry room, kitchen) I'd suggest moving them. But apart from that, no, it shouldn't be a problem.
Jenni's right, 70% is too high. You shouldn't mist the substrate every day, only when in shed.
Mele2511, if your humidity is that high I'd try to lower it, otherwise that could be detrimental to your Corn's health. 60% ambient humidity is what Royal pythons require, and 70% when in shed. But you probably know that 'cause you keep them yeah?
All the best
David