Is your employer giving you adequate breaks, with somewhere to lie down if necessary? If not then you need to speak to your line manager, as this facility should be available to you.
If your chair at the till is uncomfortable, then request another that will give you more support and comfort. Don't be afraid to ask. If you feel you need to go on leave earlier, then check out your rights on the various Government websites or various sites that offer advice to pregnant women.
www.babycentre.co.uk is pretty good for general advice.