This link may help

Here is a link that might be useful: Mexican oregano

I grow Mexican Oregano Lippia graveolens in my yard and love it. I got mine from a supplier in south Texas. You might want to contact Richters located in Canada.

I don't see any sources for plants based in the US or Canada that ship to the UK. Suggest that you contact a local nursery there, and ask them to stock some, or tell you where you can find them.