I understand that ammo marked as 9x19 NATO is loaded to a higher pressure than ammo marked 9 mm Luger.

Would 9x19 NATO be OK to use in older (pre Mk III) Hi Power pistols?

Almost all ex-SAS troopers still have both of their hands and eyes.  So yes.

I knew a London Metro detective and that’s all they shot in theirs.

The MKIIIs are rated for modern high velocity stuff but he did say "older" so no idea what that means.  I'd be leery of shooting much +P in some of those older guns even though the Euro spec ammo was hotter (heck, most ammo seems to have been hotter) and I wouldn't shoot the subgun ammo out of it.  No sense in beating up a classic older gun when there's a lot of good standard velocity ammo out there these days- well if you can find it in stock anyways.  JMHO

I went back and made an edit to my original post.

By older Hi powers I was asking about Mk III and older- I should have been more clear.

Stephen Camp did not recommend using +p 9mm in pre-94 BHPs.

>94 Mk.III's were stronger (cast frames & better steel) and could run +P fine.

+P+ is a bit too hot for BHPs.

Cast frame shoot +P's with no problem, although a slightly stiffer recoil spring is suggested, forged frame use +P minimally and most certainly upgrade the recoil spring when shooting the hotter ammo.  +P+ is really a no-go but with a cast frame likely ok with upgraded springs and very limited range use if you want it for carry ammo which is most often carried more than actually shot.

Hammer bite with a spur hammer can be a simply fix if one is inclined to round/shorten the tail end by a about the depth of 2 serrations.

Use it sparingly, it will accelerate wear.  Keep recoil spring changed, don't use a reduced-power hammer spring.

I never not use a reduced main spring in a high power.  28lbs  come on man.
The bhp isn’t as fragile as the internet wants it to be

I think all the points have been made about the older Browning HPs.  I own two from 1976 and I only shoot 115 grain FMJs through them…. Some 124 grain stuff too.  If needed, in a pinch, I’m sure they can handle a few mags of  +P or even +P+ without issues.

Honestly, I have several 9mm pistols from 1970s and well into the late 1980s and with them, I tend to only shoot plain Jane 115 grain FMJs.  My Older S&W Models 39-2 and 59s, I wouldn’t trust them with a steady diet of +P ammo due to their alloy frames, but S&W’s Model 639 and 659, that’s a different story since they’re build like tanks.  

Original Brownings are too much of a classic to beat up with hot ammo since IMHO, they’re more like fine artwork than a work horse plowing the fields.  Leave all that hard labor to modern 9mms!!

I've  always been led to believe prewar 9mm was hotter than current production commercial 9mm ammo, and that current +P is just about the same as the old stuff was......anyone else heard/read anything about that?