As far as I am aware, UK insurers don't recognise time spent with any Irish insurer. UK insurers also don't apply waiting periods to cover, but instead enforce exclusions. If a condition is excluded it's not covered at all. So in the case of your shoulder, future treatment may or may not be be covered, but in either case, your time with VHI won't be a consideration.
I can't give you any information about how necessary health insurance in the UK is, but I think that about 12/15% of the UK population have health insurance. The market is risk rated, so you pay premiums based on age.
You can find information about UK health insurance on the UK Financial Services Authority's website [broken link removed]. You can also look at the Association of British Insurer's guide to medical insurance [broken link removed].