Everyone starts somewhere. I've known a few people who went to paralegal at big international/US firms with no prior experience and absolutely killed it. If they've invited you to interview, they're not too fussed about experience levels - they've seen your CV. They'll want to see a willingness to learn and evidence that you can come up to speed quickly (these were key focuses in my recent US firm paralegal interview, even with my relatively hefty prior experience in-house and in general corporate roles). If you can show that, you'll be stood in good stead.