Thank you this is now resolved. If anyone has similar problems I switched off dial assist on my iPhone - I don’t know if that was linked. Then I spoke to Virgin help desk and the number had being showing up as an international number - and so I was being charged. With dial assist switched off and a small spending cap applied it now works and doesn’t appear to be an international number anymore. It goes through as an iMessage straightaway. I’m not sure if this due to dial assist being switched off or the £2 buffer.
Within my contract I am automatically refunded immediately should the charge trigger. I hope that explanation helps others.