You really wont see any advantage when you do this, and if you do it wrong you can really bugger up your barrel.
Take the size drill bit that you want, and find one that is 1mm smaller. Use the smaller bit to bore through the barrel. Then use the finish size bit and go through the hole again. What this does is knocks off all of the burrs and anything that could be inside of the barrel.
A good thing to also do, is to put the barrel in a lathe and get her up to speed, and lightly sand/polish the inside to make sure that you have no burrs or anything of the like.
Though again.... you really wont see any advantage to porting the barrel.
Pressurized air will act like just about anything else in the world, in that it will take the path of least resistance. That is .... it will take the easiest way to equalize out or get to its "stable" form or place.
Now look at the ports at the end of an airsoft barrel..... they are typically from .25" to 3-4" from the end of the muzzle.
Now combine the two points above...
If you port the barrel then you are going to loose any and all "push" on the bb, and will then be coasting through the rest of the barrel. And we all know that as soon as the bb is not being "pushed" it will immediately start to slow down and loose what back spin you have on the bb.
So when you port the barrel anything past ports the bb will begin to slow, as well as drag in the barrel.... both of these will kill both any hop that you have on the bb as well as any speed. So you are going to have to compensate for the loss of the two, but upping the power, as well as applying more hop.
Both of those can be done, but you are going to put more stress on the system, and run the chance of more things going wrong.
The other thing to consider is that there is a point of no return. If you make the ports to small, then you are just wasting your time. And on the flip side.... if you make them to large you are going to compromise the flight path, and power, of the bb.
Like George said above..... PDI did make a silent M24 not to long ago... and I was actually on the list to get one, but sadly I didn't "make the cut" and didn't get a rifle ( mainly because I wasn't in Japan ). Now I have thought about trying to make something like the barrel they make to see what it could do. Now these barrels I wouldn't consider to have "ports" in the since that you are talking about. These would be more along the lines of small vent holes. If a person had a water jet machine you would be able to make tiny little holes in the barrel, and you could minimize the amount of air that you loose, by using a cover and obstructing the vent holes.
But all in all... you really wont see much in the way of an advantage when it comes to porting a barrel.
Try this out if you are in the "tinkering" mood.
Have your inner barrel come to about two inches from the end of the outer barrel. And in that last two inches you can bore out some ports and then you could have a somewhat of a muzzle break. And then you could go abouts with it as you would want after that. And yes you being over in England you could go further with it, but us here in the US can't.