Hi!
I applied to Flex Legal in my second year of university looking for a part-time role but was unsuccessful (obviously as I was in full-time academia). They got back in contact when I was just out of university and I have been on their system ever since. I do not believe any experience is needed, but it did help that I had a TC and was enrolled in the LPC. I wasn't provided any training and only basic fundamentals of law were tested via some MCQs. I think if you are paired with a firm they may train you according to the role in hand.
I never got matched to a role with Flex as I never found many came my way via email - I definitely think there are a number of agencies out there who also pair paralegals with law firms (for example,
https://www.larbeyevans.com/ and
https://www.ryderreid.com/). Those agencies were always very communicative and had many roles on offer at all times.
Hope this helps
Laura