robsie,
I couldn't help with SERPS contributions as I was contracted out from that scheme. As regards NI Contributions they are really only a form of "extended income tax" and will be deducted from your salary every payday.
With the new qualifying period of 30 years for NI Contributions to enable payment of a full state pension, I cannot see the UK Government allowing people to stop paying these after 30 years while still earning above the relative minimum threshhold. It really only benefits those who for some reason could not work in the UK for enough years to qualify for a full pension under the original 39/44 years for women or men.
If you left the UK for good or at least an extended stay. and were classed by HMRC as Non-Resident then your liability to NI Contributions would cease. However along with that comes a problem - you would cease to be entitled to free treatment with the NHS.
ATM