For example:
Value of stitching knife wound: (40 QALYs) * (30,000 GBP/QALY) = 1.2M GBP
Cost of stitching knife wound: (let's say) 1000 GBP
Decision process:
Is the cost less than the value? 1000 < 1.2M, so yes. Therefore, pay for the procedure.
Whereas:
Value of expensive cancer procedure on 70-year-old: Let's say 5 QALYs just to make up something, taking into account expected lifespan and QOL and chance of remission, etc, times the same 30k GBP, = 150k GBP.
Cost of procedure: 1M GBP
Decision process:
Is the cost less than the value? 1M > 150k, so no. Therefore, don't pay for the procedure.