Not true. They are still separate levies if you like. NI gives you the right to a number of benefits which do not arise from just paying tax. The thresholds can also be slightly different and NI is calculated weekly rather than according to payroll run timings like tax is.
There is talk of merging the two. Not quite sure how that would work in practice with benefit provision or the overseas angle for people being seconded by their employers to a country we have a social security agreement with (EEA, US etc) or rather the vice versa which could be more problematic.
Of course in practice when it comes to your net pay, probably is worthwhile considering basic rate tax to be 32% factoring NI and broadly speaking once you are into higher rate the effective marginal rate being 42% given that is what will actually be taken off you.