Rest assured your carbine is a real No5 MkI. Just at a glance it's easy to tell by looking at the right side of the receiver under the rear sight. The No4 have a hump and the No5s don't. Most humpers would not go through the trouble of milling the hump off. You have a real No 5.
The downside of your carbine that hurts the value is the fact the bayonet lug was ground off. Collectors want the Jungle Carbines intact, and replacing these flash hiders is near impossible. So I'd value your No5 at around $350. Nice carbine none the less so just enjoy shooting it. By the way does it have the long, short or standard length buttstock? You can tell by looking at the top rear of the butt just ahead of the metal butt plate the will be the letter S for short, L for long and nothing for the standard length.