According to the National Institute for Health and Care Excellence (NICE) it would not usually recommend a treatment that costs more than £20,000-30,000 per QALY (quality-adjusted life year). See this link for more details on NICE's cost effectiveness formula:
https://www.nice.org.uk/proxy/?sourceurl=http://www.nice.org.uk/newsroom/features/measuringeffectivenessandcosteffectivenesstheqaly.jsp

So any drugs you have to continue taking to benefit from them won't be affected if we start living longer as it's a "per year" threshold - if it's cost effective for 1 year then it's cost effective for 100 years.

But what about one off interventions - for example surgery or short term pharmaceuticals? If a treatment could guarantee 20 years extra of full quality life then even at an eye watering half a million pounds it would still be considered cost effective.

As well as (or maybe as part of) trying to figure out how to handle an ever increasing population the UK government will have to change their guidelines for what treatments its citizens can expect to receive on the NHS.