I am size 34 dress size what size should I go for

Being a size 34, X-Small would be the most suitable choice and, if XS isn't available, Small.

How do you go to the loo in this?

Very carefully! There is a small opening in the appropriate place but this would only be suitable to use for a pee. And if you are going to wear some underwear underneath the garment, then make sure that they are small - like a thong or something similar, as you will need to pull the fabric over to one side to be able to aim correctly. It's OK though and can be done without too much hassle! Once you are inside the garment, you probably won't want to take it completely off to use the loo but that is another option. Once you get the knack of getting in and out of it, it can be done without assistance.

What size do I buy for UK 18/20?

XXLarge seems like the best option for UK 18/20

I’m a size 14-16. What size would be the best to get? Thanks

Hi, I’m a size 12 & bought a medium which is perfect. I’d think a large would be big enough, the waist is the tightest part, otherwise quite stretchy.