Cold Radiators

WEll obviously shutting down all radiators except the two in question to see if they get hotter is a good place to start.

If they don't it suggests that the flow is severely restricted. And unless the pump is cavitating I don't see how increasing system pressure will increase flow rate.

if you close off all other rads, do the "cool" ones ever get get hot?

I had a problem like this with narrow bore (10 mm) piping to a main radiator downstairs. In the end I stripped it out right back to a manifold above the ceiling, and re-plumbed it in 15 mm. I strongly suspect that there was either a kink in the pipe somewhere (it had difficult routing through 1700's oak structural timber) or a pool of solder somewhere inside a coupling, although I never located the exact cause.

Shutting off all the other rads means the two in question get nice and toasty - so this is a balancing issue? How do I go about balancing the system?

Maybe, what happens if you open up more and more of the radiators? Does one in particular "hog" all the heat when it's on? either close lockshield on the hog(s) down a bit, or open up the lockshield on the cold ones ...

If that won't work, is there any reason why you might need a higher flow rate in the whole system rather than a higher pressure? Such as lots of fan assisted radiators or a very large number of radiators. Then you can find that however you balance it you always have some relatively cool radiators. In this situation I got a standard fitting flow pump with a 25mm rather than 15mm capacity, but standard sort of pressure. E.g. 25/60

But really if it is not an unusually big or high flow system a few iteration should balance it. Just make sure all the radiators can be turned off with the lock shield valve and there isn't one jammed open near the boiler. Or an excessively big bypass pipe/towel rail or something.