It's almost come to the time where I speak to my potential supervisors for the first time, outside of emails. I'm so nervous and unsure of myself! Any tips for how to not come across as terribly shy, awkward and uncomfortable..?

They have already read 2 drafts of my proposal, said I did a really good job of taking on board their feedback, encouraged me to continue with the application process and said that they would be interested in supervising the project. They suggested meeting is the best next step after I said I'm hoping to apply for research council funding. I'm not really sure what to expect questions- or conversation-wise. I guess it's not really an interview as it's not an official uni meeting, but I'm worried I'll disappoint them and my mind will blank.

It absolutely IS an interview and you should treat it as such.

You need to try to sort the nerves. You have those nerves because you have put these PhD supervisors on a pedestal and are desperate to impress them to the exclusion of all other considerations. Remember you are interviewing them too and good candidates are hard to find. Respect your own worth and value to them. Failure to do this gets students into trouble and there's no shortage of evidence for that on this forum.

If you feel nervous keep the small-talk at the start of the conversation going for as along as possible. I find small talk makes me relate to the person and like them thus I feel more comfortable. At the start ask them side questions to get them talking, again to humanise them. I have dealt with social anxiety before and the easiest way way for me is to build a personal relationship.

It is definitely an interview but not a super stringent one. I think they will accept you if you are anyway competent, so don't worry to much.

These are very kindly-put responses, thank you :)