I think (although NI MNers may be able to tell me if I'm wrong) the situation is that NI healthcare professionals will refer NI people on to British mainland health services if the procedure/treatment they need is not easily available in NI - except in the case of terminations.
So, if you needed complex spinal surgery, your doctor in NI might happily refer you on to a hospital in the British mainland that specialises in that procedure.
But GPs and hospitals in NI cannot refer women on to NHS abortion providers in England, Wales or Scotland, although they can indicate that abortion treatment is available there. (Some detailed guidance for NI healthcare practitioners here).
And it's very difficult to just turn up in (say) Liverpool and self-refer to an NHS abortion provider. So women tend to end up paying instead - despite being UK citizens and (in theory) having exactly the same rights to NHS treatments as everyone else. (We checked this out with the HMRC and they confirmed that 'Anyone who is deemed to be ordinarily resident in the UK is entitled to free NHS hospital treatment in England.’ )
This entire area is the subject of a case currently before the Supreme Court. The judgement is expected soon but is expected to be a 'no' to funding the treatment of women from NI (although we could all be pleasantly surprised, of course.)
That's my understanding anyway, do please correct me if I'm wrong!