Brew enhancer looks like it is 50% dextrose, 25% maltodextrin and 25% pale DME. I believe Munton's Gold kits are no-sugar added kits (basically, two cans of hopped extract).
So you'd be adding more alcohol with the enhancer, a bit more body, plus a tiny bit more malt flavor. The problem is, the dextrose doesn't add either body or taste and the maltodextrin adds only body. So it will be very little flavor added.
You could add pure DME or LME and/or you could add more hops, maybe even just dry hops for aroma (which will also affect your sense of taste).
I'd stick with the kit directions. These sorts of kits shouldn't be boiled for any period of time beyond dissolving because they used hopped extract, and the hop flavor will be lessened. If you add unhopped DME/LME, you can boil that separately. You can boil hops in the added extract (or even in the brew enhancer) to get more hop flavor or bitterness.