First, let's examine whether your grappling with this guy could be (legally) regarded as self defence. Basically, whenever possible, the only form of self defence which is lawful is to remove yourself from the situation. (i.e. to run away). When this is not possible, you're entitled to use the minimum amount of force appropriate to the situation. (e.g. if possible, you should just push the attacker away from you and then run away). It's probable that the CPS would view your actions as lawful self defence as long as you were trying to use the minimal amount of force necessary to prevent injury to yourself and you didn't, for example, punch or kick the guy while he was in your headlock.
Before considering the 'intent' part of 'GBH with intent', it's necessary to consider whether the assault actually amounts to GBH. If the stab injury amounted to no more than a minor flesh wound, then a charge of 'ABH' might be more appropriate. See here to read the criteria which the CPS use when determining the appropriate assault charge:
http://www.cps.gov.uk/legal/section5/chapter_c .html
(There's no such thing as 'ABH with intent').
However, if the CPS decided that GBH is the appropriate charge, then fetching a weapon certainly constitutes 'intent', so a Section 18 charge would definitely be brought. (Again, see that link for the criteria used by the CPS).
Chris