I doubt that original made-in-Britain Mullards will last any longer. Don't forget that the so-called NOS Mullards are actually present-day production from Russia.
I find that true NOS tubes often test far stronger than their reissued counterparts. Certainly not a rule of thumb by any means, but I see it on a daily basis. As an example, just this morning I was courtesy testing a client's brand new, never before opened, straight from the factory Russian made Genalex Gold Lion KT88s to ensure they were properly matched and they each tested at 57- 58 on my TV-7 tester, where 44 is the threshold for good.
By way of comparison, I have a number of different quads of NOS GEC and NOS Genalex/GEC KT88s with production dates spanning across multiple decades and they consistently test in the mid-to-high 70s, with one quad in the mid 80s. I don't see that at all with the reissues...don't see them reaching much past the mid-60s even.
It can be a much different story when you get into the smaller tubes (e.g. 12AX7), where in my experience the Chinese tubes often test freakishly high.
NOS Mullard tubes are relatively easy to identify / authenticate. A simple google search will give you a reasonably well informed primer.