Your figures are wrong. For one the RFS fee has increased from £30 to £35. LHR-NCL is 4500 avios one way + £17.50 RFS fee (or down to 2500 avios + £17.50 RFS fee + £25 cash component).
Thanks nux. My brain wasn't working. Glad to see the OP decided in favor of the security offered by ticketing the whole thing on one ticket.